A method for quantitatively analyzing formation mechanism of ultra-deep limestone reservoir

Abstract

The application discloses a method for quantitatively analyzing formation mechanism of an ultra-deep limestone reservoir, and relates to the technical field of oil and gas exploitation. The method comprises the following steps: preparing an analysis sample; carrying out mineral and microstructure analysis, organic geochemical analysis, major and trace element analysis, isotope analysis and fluid inclusion analysis on the sample to obtain multidimensional experimental data; reconstructing a paleo-depositional environment; determining the absolute age of diagenetic events through in-situ micro-area calcite U-Pb dating, and clearly defining the reservoir pore formation period and preservation mechanism; constructing a "fluid-time-reaction" quantitative diagenetic model, establishing an evaluation standard of high-quality lithofacies, and completing the quantitative analysis of the reservoir formation mechanism. The method organically integrates various technical means such as mineral and microstructure analysis, organic geochemical analysis, major and trace element analysis, isotope analysis, fluid inclusion analysis and in-situ micro-area calcite U-Pb dating, and can significantly improve the systematic analysis ability of the limestone reservoir genetic mechanism.

Description

Technical Field

[0001] This invention relates to the field of oil and gas extraction technology, specifically a method for quantitatively analyzing the formation mechanism of ultra-deep limestone reservoirs. Background Technology

[0002] In my country's major gas-producing areas, tight rocks (including sandstone, argillaceous rocks, and argillaceous limestone) account for over 80% of natural gas reserves, possessing superior resource endowments and enormous development potential, making them a key area for ensuring stable and increased domestic natural gas production. Simultaneously, China's deep carbonate rock formations contain abundant oil and gas resources, becoming an important area for exploration. Tight carbonate oil and gas reservoir exploration technology has become a crucial direction for unconventional resource development, considered a "key area" and "highlight type" in current reserve and production growth. Many countries, represented by China and the United States, are actively increasing R&D investment to seize the commanding heights of related technologies.

[0003] Previous studies generally held that, under deep diagenetic conditions, dolomite exhibits far greater resistance to compaction and pressure solution than limestone, thus making it less susceptible to pressure solution and conducive to porosity preservation. Overpressure-induced fracturing can improve porosity and permeability, while limestone is easily pressure-dissolved, with the solution products filling and destroying pores. Therefore, under normal conditions, deep porous limestone reservoirs are difficult to form. However, recent oil and gas exploration in the Sichuan Basin has confirmed the development of a porous limestone reservoir in the second member of the Maokou Formation (Maokou Formation) of the deep Permian, breaking through the limitations of traditional reservoir geology. Currently, questions remain regarding whether the development of this type of reservoir is an isolated phenomenon or a general rule, and the geological background that allows porosity to be preserved.

[0004] Marine deep limestone reservoirs exhibit characteristics significantly different from conventional oil and gas reservoirs and unconventional shale gas reservoirs: frequent structural variations, strong heterogeneity, extremely low porosity (<2%) and permeability (<0.1mD), complex reservoir space, predominantly self-generated and self-storage type reservoir assemblages, and multi-layered, multi-source, and omnidirectional hydrocarbon expulsion and charging characteristics. Oil and gas enrichment patterns are complex, with extremely high differences in gas production from well to well, making distribution prediction very difficult. These unique geological conditions result in extremely low applicability of conventional and unconventional oil and gas reservoir exploration and prediction technologies, making it difficult to effectively evaluate and accurately predict this type of reservoir. Therefore, there is an urgent need to independently develop an efficient exploration, development, and prediction technology system for marine limestone tight gas adapted to the complex geological conditions of the Sichuan Basin. The most critical and primary task is to establish an effective method to reveal the formation mechanism of limestone reservoirs, providing theoretical basis and technical support for subsequent exploration deployment and development strategies.

[0005] The source rock and reservoir evaluation criteria established based on conventional oil and gas and shale gas identification indicators (such as depth, thickness, organic matter abundance, and maturity) have poor applicability in marine deep limestone reservoirs. Furthermore, existing technologies for deep limestone reservoirs often focus on single techniques (such as relying solely on geochemistry or microstructure), lacking a systematic quantitative model integrating multiple technologies, resulting in incomplete mechanism analysis and low prediction accuracy. To address this issue, our project team developed a collaborative evaluation method applicable to this type of reservoir based on the "source-reservoir-caprock-reservoir" theory. This method focuses on characteristics such as "large thickness, wide distribution, high TOC content, high maturity, and low brittleness index." Through formation evolution and source tracing analysis, it revealed the reservoir formation mechanism and organic matter enrichment patterns, reconstructed the formation patterns of different lithofacies, expanded the new field of unconventional exploration in high-salinity marine environments, achieved a key breakthrough in high-quality lithofacies evaluation, and promoted theoretical innovation from single-factor analysis to multi-factor, interconnected source tracing.

[0006] Furthermore, calcium isotope geochemistry methods have shown significant potential in revealing the diagenesis and pore evolution mechanisms of limestone reservoirs. Calcium isotopes (δ¹²) 44 / 40 As an effective diagenetic tracer, carbon (Ca) can systematically identify the contributions of diagenetic fluids from different sources (such as seawater, atmospheric freshwater, and hydrothermal fluids), clarify the spatiotemporal distribution characteristics of pressure solution and cementation, and thus reconstruct the paleoenvironmental conditions (such as temperature, salinity, and pH) of key diagenetic stages. Combined with carbon and oxygen isotope, trace element, and petrographic analysis, it can further reveal the genesis of reservoir heterogeneity and hydrocarbon charging pathways, providing quantitative constraints on the formation mechanism of deep limestone "self-generated and self-storage" gas reservoirs. This overcomes the limitations of traditional geochemical indicators in complex diagenetic systems and provides a new technical approach and theoretical support for establishing a quantitative evaluation system applicable to the formation mechanism of tight reservoirs in marine limestone. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of the prior art and to propose and implement for the first time a systematic and multi-dimensional quantitative analysis method for the formation mechanism of ultra-deep limestone reservoirs.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0009] A method for quantitatively analyzing the formation mechanism of ultra-deep limestone reservoirs includes the following steps:

[0010] S1, core samples were selected from ultra-deep limestone reservoir sections for observation and sampling, and analytical samples were prepared;

[0011] S2, mineral and microstructure analysis, organic geochemical analysis, major and trace element analysis, isotope analysis, and fluid inclusion analysis are performed on the sample to obtain multi-dimensional experimental data; wherein, the isotope analysis includes calcium isotope δ 44 / 40 Ca analysis;

[0012] S3, based on experimental data, reconstructs the paleosedimentary environment, quantitatively characterizes the reservoir space features, and analyzes the properties and evolution history of diagenetic fluids;

[0013] S4, the absolute age of the diagenetic event was determined by in-situ micro-area calcite U-Pb dating, combined with calcium isotope δ¹⁴ dating. 44 / 40 Ca data clarifies the formation period and preservation mechanism of reservoir porosity;

[0014] S5, construct a quantitative diagenetic model of "fluid-time-reaction", establish a high-quality lithofacies evaluation standard, and complete the quantitative analysis of reservoir formation mechanism.

[0015] Furthermore, in step S2, the mineral and microstructure analysis is performed using one or more of the following techniques: polarizing microscope, high-resolution scanning electron microscope, scanning electron microscope (SEM), X-ray diffraction (XRD), cathodoluminescence (CL), and CT scanning.

[0016] Furthermore, the isotope analysis in step S2 also includes carbon and oxygen isotope analysis, strontium isotope analysis; the calcium isotope δ... 44 / 40 Ca analysis was performed using a multi-receiver inductively coupled plasma mass spectrometer to trace the fluid source and the water-rock interaction process.

[0017] Furthermore, the organic geochemical analysis described in step S2 includes TOC detection, saturated hydrocarbon chromatography-mass spectrometry analysis, and biomarker compound analysis to clarify the abundance, type, maturity, and source of organic matter.

[0018] Furthermore, in step S3, paleosalinity, redox environment, and paleoproductivity are reconstructed using major and trace elements, rare earth elements, and biomarker compounds, revealing the mechanism of organic matter enrichment.

[0019] Furthermore, in step S4, by comparing the calcite U-Pb dating results with the formation time of the limestone, it is determined whether the pores were formed during the sedimentary or diagenetic period; and this is combined with the calcium isotope δ¹⁴Tb dating results. 44 / 40 By studying the variation characteristics of Ca data, we can identify the initial carbonate mineral transformation process and analyze the porosity preservation mechanism.

[0020] Furthermore, the quantitative diagenetic model described in step S5 uses paleotemperature and paleopressure data measured by fluid inclusions, along with calcium isotope and rare earth element characteristics, to achieve quantitative characterization of the diagenetic evolution process.

[0021] Furthermore, in step S5, the evaluation criteria for high-quality lithofacies include the abundance of organic matter, the maturity of organic matter, terrigenous input indicators, the degree of development of nanoscale pores in reservoir space, and geochemical indicators of diagenetic cementation strength.

[0022] Furthermore, in step S2, the major and trace element analysis includes the determination of Sr, Mg, Fe, Mn and rare earth elements, the paleosalinity is determined by the Sr / Mg ratio, and the paleoredox conditions are inverted by the Fe / Mn ratio.

[0023] Furthermore, in step S4, cathodoluminescence images are used to identify the calcite growth sequence, and combined with the differences in Fe and Mn content, cement formed in reducing burial environments and oxidizing near-surface diagenetic environments is distinguished.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] (1) This invention proposes and implements for the first time a systematic and multi-dimensional quantitative analysis method for the formation mechanism of ultra-deep limestone reservoirs. This method organically integrates various technical means such as mineral and microstructure analysis, organic geochemical analysis, major and trace element analysis, isotope analysis, fluid inclusion analysis, and in-situ micro-area calcite U-Pb dating, forming a complete research system of "sedimentary environment reconstruction - diagenetic process analysis - pore evolution quantification - high-quality reservoir evaluation", which significantly improves the systematic analysis capability of the genetic mechanism of limestone reservoirs.

[0026] (2) This invention introduces the calcium isotope δ 44 / 40 Using calcium (Ca) as a key tracer, combined with U-Pb dating technology, we were able to accurately identify the source, evolution path, and porosity formation period of diagenetic fluids. Through synergistic constraints of multiple parameters, including calcium isotopes, carbon and oxygen isotopes, strontium isotopes, and rare earth elements, we can effectively distinguish the porosity genesis between sedimentary and diagenetic periods, reveal the initial mineral transformation process and porosity preservation mechanism, and overcome the limitations of traditional geochemical methods in complex diagenetic systems.

[0027] (3) This invention constructs a quantitative diagenetic model of "fluid-time-reaction" and establishes a high-quality lithofacies evaluation standard applicable to marine ultra-deep limestone reservoirs. Through multi-source data fusion and model-based characterization, it realizes the leap from qualitative description to quantitative analysis of reservoir formation mechanism, providing a reliable theoretical basis and technical support for exploration deployment, favorable area prediction and development strategy formulation of similar reservoirs, and has important practical application and promotion value. Attached Figure Description

[0028] Figure 1 This is a flowchart illustrating the method for quantitatively analyzing the formation mechanism of ultra-deep limestone reservoirs in this invention. Detailed Implementation

[0029] The present invention will be further described below with reference to the accompanying drawings and embodiments. The embodiments of the present invention include, but are not limited to, the following embodiments.

[0030] Unlike conventional oil and gas resources, which are limited in distribution and small in quantity, marine limestone oil and gas resources are characterized by wide exploration areas and large quantities, possessing enormous resource potential and economic value, and have received high attention from the global oil and gas industry and governments worldwide. However, given the challenges of efficient exploration, development, and prediction of marine limestone oil and gas resources under complex geological conditions, as well as the significant variations in gas production from well to well, there is an urgent need to establish a method for quantitative analysis of the formation mechanism of deep limestone reservoirs. The indoor testing and analysis methods proposed in this invention are as follows:

[0031] Polarizing microscope: Used for observing the microstructure and identifying the mineral composition of ultra-deep limestone. The DM4500P polarizing microscope manufactured by Leica GmbH, Germany, purchased by the National Key Laboratory of Oil and Gas Reservoir Geology and Development Engineering, meets the needs of this project.

[0032] High-resolution scanning electron microscopy (SEM): used to observe the ultra-microscopic morphology and structure of minerals in dolomite and gypsum-salt rocks. The National Key Laboratory of Oil and Gas Reservoir Geology and Development Engineering has purchased a Quanta 250 FEG field emission scanning electron microscope manufactured by FEI Corporation of the United States. Its resolution can reach up to 1.2 nm, the maximum magnification can reach 1 million times, and it can perform real-time imaging, which can meet the research needs of this project.

[0033] Cathodoluminescence (CL) is used to identify the cementation sequence, diagenetic sequence, and diagenetic fluid properties of carbonate rocks. By observing the differences in luminescence of different diagenetic minerals under electron beam bombardment, it effectively distinguishes cements from multiple stages, such as calcite and dolomite. The CathodoLuminescence Model CCL 8200 mk5 cathodoluminescence emitter, manufactured by Cambridge Image Technology Ltd. in the UK and purchased by the State Key Laboratory of Oil and Gas Reservoir Geology and Development Engineering, can be coupled with existing polarized microscopes, offering high resolution and clear images, thus meeting the research needs of this project.

[0034] X-ray diffraction (XRD) is used to identify the mineral composition of different ultra-deep limestone facies and to clarify the variations in the content of certain elements (Fe, Mn, etc.). The Rigaku D / Max IIIC X-ray diffractometer, purchased by the National Key Laboratory of Oil and Gas Reservoir Geology and Development Engineering from Japan, has successfully served multiple research projects, demonstrating high testing accuracy.

[0035] Electron probe microanalysis (EPMA): This method can obtain micro-area morphology images, corresponding compositions, and quantitative analysis of constituent elements in the survey area. The National Key Laboratory of Oil and Gas Reservoir Geology and Development Engineering has purchased the latest EPMA-17200H Series electron probe microanalyzer manufactured in Japan. Its elemental analysis range includes: 5 B- 92 U, analytical precision: for major elements (content > 5%), it can reach ≤ 1%.

[0036] Major and trace elements: These can be used to study ancient sedimentary environments, element migration patterns, and property identification, including rare earth elements. The National Key Laboratory of Oil and Gas Reservoir Geology and Development Engineering is equipped with TOF-SIMS, which can meet the research needs of this project.

[0037] Stable isotope analysis: used to analyze paleoenvironments and trace carbon sources, and to determine fluid properties and origins. The MAT 253 plus isotope mass spectrometer purchased by the National Key Laboratory of Oil and Gas Reservoir Geology and Development Engineering can meet the needs of C, O, and Sr isotope determination in this project.

[0038] Biomarker compounds: These identify sedimentary environments and the sources of organic matter, clarifying the patterns of organic matter enrichment, thereby enabling the study of the genesis of high-quality lithofacies. The ISQ gas chromatography-mass spectrometry system manufactured by Thermo Fisher Scientific, USA, purchased by the State Key Laboratory of Oil and Gas Reservoir Geology and Development Engineering at Southwest Petroleum University, can support this research.

[0039] TOC (Total Organic Carbon) analysis is used to determine the organic matter abundance in ultra-deep marine limestone facies, thus identifying the basic characteristics of hydrocarbon generation conditions in ultra-deep limestone. The CS230SH organic carbon and sulfur analyzer manufactured by Leco Corporation of the United States, purchased by the National Key Laboratory of Oil and Gas Reservoir Geology and Development Engineering at Southwest Petroleum University, provides support for this work.

[0040] Fluid inclusion thermometry: Determines paleotemperature and paleopressure, which, together with other parameters, constrain the nature and origin of diagenetic fluids.

[0041] In-situ micro-area calcite U-Pb dating: Determines the time of fluid evolution and clarifies whether the reservoir formed during the sedimentary or diagenetic period.

[0042] CT scanning: Used for non-destructive characterization of the three-dimensional pore structure, fracture network, and connectivity within ultra-deep limestone samples, enabling precise quantification of parameters such as porosity and pore size distribution. The Xradia 510 Versa three-dimensional X-ray microscopy system, manufactured by ZEISS GmbH of Germany, purchased by the State Key Laboratory of Oil and Gas Reservoir Geology and Development Engineering, boasts a spatial resolution of up to 0.7 μm, enabling high-contrast, high-resolution scanning over long working distances, and in-situ mechanical loading capabilities, thus meeting the research needs of this project.

[0043] Ca isotopes: used to accurately trace fluid sources, water-rock interactions, and global biogeochemical cycles during carbonate sedimentation and diagenesis. High-precision mass spectrometry is used to determine δ¹⁸O₂. 44 / 40 Ca values ​​provide crucial geochemical evidence for ultra-deep fluid activity (such as hydrothermal vents and atmospheric water leaching). The National Key Laboratory of Oil and Gas Reservoir Geology and Development Engineering has acquired a Neptune Plus multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) manufactured by Thermo Fisher Scientific, Germany. Equipped with a membrane desolvation sample introduction system, it can achieve high-precision Ca isotope determination (external reproducibility better than ±0.1‰, 2SD), which meets the research needs of this project.

[0044] The research approach and advantages of this invention are as follows:

[0045] (1) Formation of porosity in limestone reservoirs:

[0046] The characteristics of seawater can be determined by major and trace elements, rare earth elements, inorganic CO isotopes, and Sr isotopes, clarifying whether it is from the same seawater source, thereby determining whether the pores of the limestone reservoir were formed during the sedimentary or diagenetic period. Then, in-situ micro-area calcite U-Pb dating analysis is performed in the limestone reservoir to determine the formation time of calcite in the pores, thereby verifying whether the reservoir was formed during the sedimentary or diagenetic period: if the formation time of calcite in the pores is newer than that of the limestone, then the pores were formed during the diagenetic period, mainly due to the influence of diagenesis, and formed by the precipitation of calcite fluids after entering; if the calcite U-Pb dating time in the pores is close to that of the limestone reservoir, then it was formed during the sedimentary period.

[0047] The reservoir porosity formed during different stages of sedimentation and diagenesis exhibits significant differences in its reservoir formation mechanism, which can clarify its main controlling factors: for example, the main controlling factor for limestone reservoirs during diagenesis is the dissolution effect of organic acids or diagenetic fluids during the burial period, which forms pores as oil and gas reservoirs through dissolution; while the pores of reservoirs during sedimentation are mainly formed and preserved by syn- and quasi-syn-sedimentary dissolution, which then forms oil and gas reservoirs after oil and gas accumulation.

[0048] (2) Preservation of porosity in limestone reservoirs:

[0049] First, deep abnormal high pressure can effectively prevent the compaction of strata during burial, thus preserving porosity. Second, the discharge of fluids from source rocks, natural gas injection, crude oil cracking, and sealing diagenesis can also lead to the formation of abnormal high pressure. In addition, the transformation of initial carbonate minerals can also change the ability of carbonate sediments to resist compaction and preserve original porosity. For example, during the sedimentation-diagenesis process, minerals change from aragonite to high-magnesium calcite and then to calcite, and their stability and resistance to compaction gradually increase. The degree of preservation of original porosity also increases with the increase of the minerals' resistance to compaction.

[0050] To determine whether sedimentary minerals play an important role in the preservation of porosity in limestone reservoirs, the initial carbonate mineral transformation can be identified and determined using high-resolution scanning electron microscopy and Ca isotope analysis. This allows for the study of whether the formation and preservation of limestone reservoir porosity are related to initial sedimentation and the transformation of sedimentary carbonate minerals.

[0051] Based on the above research approach, the present invention will be described in detail below:

[0052] (1) Study on the basic characteristics and sedimentary evolution of marine ultradeep limestone

[0053] First, core samples were taken from the selected limestone reservoir section to be analyzed. Thin sections were then prepared and microscopically observed using a high-resolution scanning electron microscope (SEM) with argon-ion polishing, revealing the detailed composition and microstructural characteristics of the minerals in the limestone, and accurately identifying mineral types and their distribution. Based on the preliminary identification of mineral composition, lithofacies identification was performed, and their vertical distribution patterns were analyzed. Combined with XRD analysis of mineral composition, the main lithofacies types of the limestone reservoir were determined. Based on core samples from multiple wells, the planar distribution characteristics of various lithofacies in the limestone reservoir were preliminarily identified. Finally, CT scans were used to quantitatively analyze physical properties such as porosity, pore size distribution, and pore connectivity.

[0054] Furthermore, major and trace elements are used to identify paleoenvironmental characteristics during the deposition of ultra-deep limestone, including: determining the original mineral facies and paleosalinity through Sr and Mg contents and their ratios; retrieving paleoredox conditions using Fe and Mn contents and the Fe / Mn ratio; and tracing paleowater properties and provenance inputs using rare earth element (REE) distribution models. By combining these major and trace element indicators with petrographic observations, paleosalinity, redox states, and paleomarine chemical environments during the deposition of ultra-deep limestone can be comprehensively reconstructed, providing sedimentary basis for the genetic analysis of high-quality reservoirs.

[0055] To further improve the accuracy of paleoenvironmental reconstruction and deepen our understanding of diagenetic mechanisms, this method introduces calcium isotopes (δ¹²C). 44 / 40(Ca) Geochemical indicators. Calcium isotopes are sensitive to the chemical composition of seawater, the properties of diagenetic fluids, and the diagenetic processes of carbonate rocks, and can effectively trace paleooceanic calcium cycles, diagenetic environmental evolution, and porosity preservation mechanisms. For example, by analyzing δ¹⁸O⁻ in limestone... 44 / 40 The vertical variation characteristics of calcium (Ca), combined with its correspondence with traditional salinity indicators (such as the gammacerane index and Sr / Mg ratio), can identify paleosea salinity evolution and water stratification events during sedimentary periods, providing independent geochemical evidence for paleoenvironmental reconstruction. By combining carbon and oxygen isotopes, major and trace elements, and rock texture characteristics, calcium isotope analysis can provide independent constraints on paleosalinity, paleotemperature, water stratification, and fluid activity during diagenesis, thereby deepening our understanding of the formation and preservation environment of limestone reservoirs.

[0056] Subsequently, the characteristics of organic matter were determined using saturated hydrocarbon chromatography and mass spectrometry of different types of ultra-deep limestone lithofacies. The results of saturated hydrocarbon chromatography and mass spectrometry analysis of the limestone were obtained. In the chromatograms, nC... 17 Adjacent to Pr, nC 18 Features adjacent to Ph are used to identify the carbon number of n-alkanes; then, the carbon number distribution characteristics of n-alkanes are used for preliminary identification of the source of organic matter, when the carbon number is concentrated in nC. 20 The following generally represent organisms primarily composed of aquatic life (mainly algae and lower aquatic organisms), nC 23 -nC 25 Mainly represented by emergent plants, nC 27 -nC 29 Representing primarily terrestrial higher plants (trees), with a value greater than nC. 31 It mainly represents terrestrial herbaceous plants. Based on the above index range, determine whether the organic matter belongs to terrestrial or marine facies.

[0057] Using Pr / C in biomarker compounds 17 With Ph / C 18 Preliminary environmental identification was conducted. The reducing power ratio (Pr / Ph) was used for initial identification: a Pr / Ph ratio less than 0.5 reflected a strongly reducing environment; a Pr / Ph ratio between 0.5 and 2 indicated a reducing environment; and a Pr / Ph ratio greater than 2 indicated an oxidizing environment with terrestrial organic matter input. Subsequently, the Pr / nC ratio was used... 17 With Ph / nC 18 The ratio of mass to charge ratio is used to identify the specific source of organic matter. Two mass-to-charge ratio conditions were selected for analysis in the mass spectra: m / z=217 and m / z=191. In the saturated hydrocarbon mass spectrometry analysis results at m / z=191, the gammacerane index (GI) was used as an indicator of water salinity. The calculation method is GI = gammacerane / C 30 Horwathane. Gammacerane is a non-horwathane series C12C ... 30- Triterpenoids, the gammacerane index is closely related to the salinity and water stratification of the water body during organic matter deposition. Source rocks from freshwater sedimentary environments typically have lower gammacerane content, while those from saline water sedimentary environments typically have higher content. The gammacerane index (gammacerane / C) is... 30 Steranes (Ch) are generally close to or greater than 1. In the saturated hydrocarbon mass spectrometry analysis results at m / z=217, the composition and distribution characteristics of steranes can well reveal the evolutionary history of source rocks and sedimentary environments: Ch 27 R-steranes are mainly derived from lower aquatic organisms such as algae, C 28 R-steranes are mainly derived from terrestrial higher plants, C 29 R-steranes are mainly associated with cyanobacteria. The distribution characteristics of the three steranes can intuitively determine the source of organic matter and thus the sedimentary environment (marine facies generally have an L-shaped morphology, while terrestrial facies generally have a V-shaped morphology). For example, in the mass spectrum of biomarker compounds in the third section of the Leikoupo Formation in the Sichuan Basin, regular steranes show an "L" shape, indicating a marine origin.

[0058] In summary, a system based on mineral content analysis, organic matter characteristics, organic matter maturity (Ro) analysis, major and trace elements (terrigenous input indicators Al and Ti, redox indicators U / Th, V / (V+Ni), V) was established. EF U EF A comprehensive analytical system was developed, incorporating U / Al and Mo / Al ratios, paleoproductivity indicators such as P, Ba, Zn+Cu+Ni, Ba / Al and Zn / Al ratios, chemical weathering index (CIA), nitrogen, and organic carbon isotopes. This system reconstructed the paleoclimate and paleohydrological changes during the formation of marine tight limestone, including arid climate, high salinity, water stratification, and reducing hydrological conditions. This further clarified that the argillaceous limestone is a high-quality, organic-rich lithofacies within marine tight limestone. Based on this, and considering data from multiple aspects such as mineral types and their contents, TOC content, organic matter maturity, elemental and isotopic composition, and nanopore development, evaluation criteria for this high-quality, organic-rich lithofacies were defined: high maturity (typically Ro > 1.2), low terrigenous input (Al content < 5%, Ti content < 0.4%), type I-II1 kerogen, average TOC > 1.0, and highly developed nanopores. This represents a key breakthrough in the evaluation of high-quality lithofacies in marine tight limestone, promoting a theoretical innovation in the evaluation system from single-factor analysis to multi-factor, interconnected tracing. This not only enriches and improves the reservoir development model of unconventional carbonate rocks, but also opens up new exploration ideas for unconventional carbonate rocks, providing important theoretical guidance for the fine characterization of marine tight limestone reservoirs and the prediction of reservoir facies zones.

[0059] (2) Sedimentary-diagenetic evolution path of marine ultradeep limestone reservoirs

[0060] Limestone samples underwent sample pretreatment and chemical separation, and the prepared samples were then analyzed using a cathodoluminescence microscopy (CL) system. Bright orange-yellow calcite regions were observed in the CL images, typically indicating high Mn content. 2+ Low Fe 2+ It may have formed in a reducing diagenetic environment (such as a burial environment); areas of calcite that are non-luminescent or dimly luminescent usually indicate low Mn content. 2+ / High Fe 2+ Calcite may form in oxidizing or near-surface diagenetic environments (such as atmospheric water or seawater environments). By identifying different luminescent generations, the growth sequence and diagenetic evolution history of calcite can be established.

[0061] Calcium isotope analysis was used to identify and determine the mineral transformation of limestone samples and to trace the fluid origin: calcium isotope data were compared with major and trace elements (such as Sr, Mg, Fe, Mn), rare earth elements, and carbon and oxygen isotopes (δ¹⁸O). 13 C、δ 18 O), strontium isotopes ( 87 Sr / 86 Combining Sr data with other data helps determine whether seawater characteristics originate from the same period, thereby identifying the formation period (depositional or diagenetic) of limestone reservoir porosity. Employing multiple high-precision techniques to quantitatively identify diagenetic evolution paths and improve identification accuracy is crucial for quantitative analysis of reservoir formation mechanisms.

[0062] Using calcium isotopes (δ 44 / 40 (Ca) can identify the original mineral composition (aragonite and calcite), distinguish the source of diagenetic fluids (seawater, atmospheric water, hydrothermal), assess the openness of diagenetic systems (open and closed), and reconstruct the evolutionary history of diagenetic fluids. During precipitation, aragonite is significantly enriched in light calcium isotopes (Ca) than coexisting fluids. 40 Ca), which means that the δ of aragonite 44 / 40 The Ca value is about 1.0‰ to 1.5‰ lower than that of calcite; compared to aragonite, calcite is more enriched in heavy calcium isotopes ( 44 Ca), its δ 44 / 40 The Ca value is higher. Calcium isotope (δ) 44 / 40 A low δ¹⁴C value may indicate strong chemical fractionation, fluid-rock interactions, or specific biological processes; hydrothermal fluids typically possess unique isotopic values. In open systems with a continuous fluid supply, the fluid's calcium isotopic composition remains stable, so all carbonate minerals precipitated in this system have a constant δ¹⁴C value. 44 / 40 Ca value; in a closed system, the fluid volume is limited, and with carbonate minerals (preferentially binding heavy isotopes)44 The precipitation of Ca will cause the remaining fluid to become increasingly enriched with light calcium isotopes (Ca). 40 Ca), which causes later-precipitated minerals to have higher δ values ​​compared to earlier-precipitated minerals. 44 / 40 The Ca value gradually decreases. Therefore, calcium isotopes act like sensitive "recorders," recording information through their fractionation behavior, combining major and trace elements (such as Sr, Mg, Fe, Mn), rare earth elements, and carbon and oxygen isotopes (δ¹⁰). 13 C、δ 18 O), strontium isotopes ( 87 Sr / 86 Indicators such as Sr form a complete chain of evidence, reconstructing the evolutionary history of diagenetic fluids.

[0063] δ 18 A low O value often indicates atmospheric water leaching or a high-temperature environment; δ 13 A lower C value may be related to organic matter degradation or hydrocarbon involvement; the carbon and oxygen isotopes of marine primary limestone have a specific range; strontium isotopes ( 87 Sr / 86 Sr content analysis can determine whether a fluid originates from a strontium-rich terrestrial silicate region (e.g., high Sr ratio) or is associated with contemporaneous seawater (low Sr ratio, close to contemporaneous seawater values). Combining Sr content analysis with other methods can more accurately determine the fluid's origin (marine, terrestrial, or deep-source). High Sr content (e.g., >1000 ppm) typically indicates well-preserved original sedimentary structures and weak diagenetic alteration; conversely, low Sr content indicates strong surface diagenetic alteration. Marine-sourced fluids typically have low Fe and Mn contents and high Fe / Mn ratios (seawater has a very high Fe / Mn ratio), therefore marine calcite cements usually do not luminescent. Atmospheric water or buried-sourced fluids can leach Fe from nearby silicate minerals (e.g., clay). 2+ and Mn 2+ If the fluid is in a reducing state, the resulting calcite will be rich in Fe and Mn, and will exhibit bright cathodic luminescence. Hydrothermal fluids are typically characterized by high Fe and high Mn content because the high temperature increases the activity and solubility of the elements; therefore, the resulting hydrothermal calcite or dolomite often displays bright cathodic luminescence. High Mn content sometimes also indicates the input of terrigenous material, as continental rock weathering is an important source of Mn.

[0064] Different fluids and environments exhibit drastically different REE distribution patterns:

[0065] ① Marine fluids / seawater: HREE-enriched, exhibiting a steep right-sloping pattern with lower values ​​on the left and higher values ​​on the right, and a significant negative Ce anomaly (Ce / Ce* < 0.9; Ce...). SN / (0.5La SN +0.5Pr SNThis is because the marine oxide layer will transfer Ce 3+ Oxidized to Ce 4+ And remove. A high Y / Ho ratio (>44) is a hallmark of seawater. If a carbonate rock sample retains the REE pattern of seawater, it indicates that its diagenetic alteration was weak and that it has largely preserved the original sedimentary information.

[0066] ② Atmospheric Freshwater: Flat or slightly enriched with LREE. Due to the lack of complex complexation processes found in seawater, differentiation is not significant, with no Ce anomaly or only a very weak negative anomaly (Ce / Ce*≈1). This is because atmospheric freshwater systems are usually in an oxidized state, but Ce removal efficiency is much lower than in marine environments. The Y / Ho ratio is low (close to shale values ​​~27). The presence of an atmospheric freshwater pattern indicates that the rocks have undergone atmospheric hydroforming processes, such as atmospheric water leaching, dissolution, and recementation.

[0067] ③ Hydrothermal fluids: A typical characteristic is MREE-enrichment, exhibiting a "hump" shape. This is because MREE (such as Sm, Eu, Gd) complexes are more stable at high temperatures; significant positive Eu anomalies (Eu / Eu) are frequently observed. * >1.1;Eu / Eu*=Eu SN / (0.5Sm SN +0.5Gb SN This is Eu under high temperature (>250°C) reduction conditions. 2+ Separation is a hallmark of hydrothermal activity. This pattern indicates hydrothermal fluid activity, such as hydrothermal dolomitization, hydrothermal dissolution, or mineral infilling, processes crucial for the formation of high-quality reservoirs.

[0068] In addition to determining the properties of diagenetic fluids, REE partitioning models can also provide important target areas for U-Pb dating.

[0069] (3) Method for constructing a quantitative model for the formation of marine ultra-deep high-quality limestone reservoirs

[0070] When quantitatively studying the genetic mechanism of ultra-deep limestone reservoirs, two main directions need to be grasped: quantitative analysis of the main controlling factors and quantitative analysis of the evolution process.

[0071] Quantitative analysis of controlling factors involves correlation analysis of sedimentary factors (mineral composition, TOC value, organic matter type), diagenetic factors (fluid temperature, fluid source, organic evolution), and reservoir quality obtained from CT scans. Quantitative analysis of the evolutionary process aims to understand the diagenetic evolution history. This requires using fluid inclusions to determine paleotemperature and paleopressure (homogenization temperature indicates diagenetic temperature, freezing point temperature infers salinity), reconstructing pressure evolution history through population analysis, and revealing fluid properties through compositional analysis. Combining fluid inclusion temperature-salinity data and rare earth element (REE) characteristics, calcium isotopes provide independent chemical kinetic constraints. These, along with calcite U-Pb dating results, are used to construct a quantitative "fluid-time-reaction" diagenetic model, significantly improving the accuracy of reservoir formation mechanism analysis and synergistically constraining the properties and sources of diagenetic fluids. Finally, in-situ micro-area calcite U-Pb dating is used to combine the obtained age data with the above analysis results to determine the formation time of calcite in the pores, clarify its geological significance, and thus verify the formation period of reservoir pores (depositional or diagenetic period).

[0072] The following is a specific embodiment for further detailed explanation:

[0073] This embodiment takes the marine ultra-deep limestone of the Leikoupo Formation in the Sichuan Basin as an example. Based on petrological, sedimentological, and geochemical studies, it comprehensively utilizes various techniques, including cathodoluminescence, CT scanning, XRD experiments, major and trace element analysis, rare earth element analysis, carbon, oxygen, and strontium isotope analysis, TOC testing, biomarker compounds, U-Pb dating, and Ca isotope analysis, to explore and summarize the sedimentary and diagenetic characteristics of the ultra-deep limestone. It quantitatively constructs the sedimentary-diagenetic evolution path and accurately and effectively identifies the regulation of sedimentary-diagenetic processes on limestone reservoir formation. The technical route is as follows: Figure 1 As shown. Specifically:

[0074] This embodiment takes the marine ultra-deep limestone reservoir of the Leikoupo Formation III (hereinafter referred to as Leikoupo III) in the Sichuan Basin as the research object, and adopts the technical route of comprehensive experimental analysis to systematically carry out quantitative characterization of the reservoir sedimentary-diagenetic evolution process and reservoir performance.

[0075] 1. Mineral composition and organic matter characteristics

[0076] This study focuses on the marine ultra-deep limestone reservoir of the Leikoupo Formation III (hereinafter referred to as "Leikoupo III") in the Sichuan Basin. Representative core samples were selected from well HC125 in Leikoupo III. Based on well logging data and thin section analysis, the lithological characteristics were preliminarily clarified, and petrological and geochemical experiments were systematically conducted. Through TOC testing, saturated hydrocarbon chromatography-mass spectrometry, kerogen microstructure analysis, and carbon isotope analysis, the lithological characteristics of the Leikoupo III were identified. 2 Subsegment kerogen is predominantly of type I–II1, δ 13The carbon (C) content ranged from -31‰ to -27‰, with an average of -28.86‰, indicating good hydrocarbon generation potential. Microscopic examination of the kerogen revealed that it was predominantly composed of amorphous saprophytic matter, followed by amorphous humic matter, generally of type I–II1, with some localized presence of type II2. The carbon isotopes of the kerogen ranged from -31‰ to -27‰, with an average of -28.86‰, further confirming its predominance of type I–II1. TOC analysis showed that... 2 The average TOC of the source rocks in this sub-section is 0.8%, with the highest TOC content (0.53%) found in the argillaceous limestone within the carbonate rocks, all indicating relatively good source rocks. Measured Ro values ​​range from 1.01% to 1.73%, with an average of 1.30%, indicating that the source rocks in this area are in a highly mature condensate-wet gas generation stage. Saturated hydrocarbon chromatography-mass spectrometry results show that n-alkanes exhibit a single peak, with the main peak located at C10. 17 –C 20 It exhibits a significant advantage in low carbon number; the average Pr / Ph ratio is 0.53, reflecting a typical marine, slightly reducing environment; the average CPI and OEP are 1.10 and 1.09, respectively, further confirming its high maturity characteristics. The steranes and terpenes have similar overall compositions, exhibiting high content of long-chain tricyclic terpenes, high Ts / Tm values, and high gammacerane content, indicating a high-salinity, reducing environment; regular steranes show a C... 27 >C 29 >C 28 The "L"-shaped distribution reflects that the organic matter mainly comes from marine algae and bacteria.

[0077] X-ray diffraction (XRD) analysis results show that the calcium content in the Lei 3 Member marl ranges from 26% to 98.7%, with an average of 70.75%, and TOC is significantly negatively correlated with calcium content. The clay mineral content ranges from 0% to 66%, with an average of 17.6%, and TOC is positively correlated with clay content, possibly indicating that clay plays a controlling role in the adsorption and enrichment of organic matter. Vertically, the argillaceous limestone section gradually transitions to marl and micritic limestone sections, accompanied by a decrease in pyrite, quartz, and clay content and an increase in carbonate rock content, which shows a good positive correlation with TOC.

[0078] 2. Reconstruction of Paleosedimentary Environment and Organic Matter Enrichment Mechanism

[0079] Based on elemental geochemical indicators, the paleosedimentary environment characteristics of the Lei 3 Member were reconstructed. Major and trace element analysis, combined with rare earth element (REE) and redox-sensitive elements (Fe, Mo, U, V, etc.), showed that V EF Values ​​range from 0.5 to 1.5, with an average of 2; U EFThe values ​​ranged from 0 to 20, with an average of 10; the V / (V+Ni) ratio ranged from 0.65 to 0.9, with an average of 0.75; and the U / Th ratio ranged from 0.5 to 2.5, with an average of 1.5. These indicators collectively suggest that the Lei 3 section was in an anoxic-anaerobic environment, transitioning longitudinally from a strongly reducing environment to an anoxic environment. Paleoproductivity showed a significant positive correlation with TOC. The paleoproductivity index P... XS Ba XS and Ni XS +Cu XS +Zn XS The average values ​​are 0.25, 242, and 221, respectively, decreasing gradually from bottom to top. In contrast, δ... 15 N org (‰; 2.55~4.85‰) and δ 13 C org The indicators (‰; -27.07~-31.07‰) gradually increased. These indicators all suggest that marine paleoproductivity decreased progressively during the limestone deposition period. The enrichment of organic matter was mainly controlled by the coupling effect between the anoxic environment and paleoproductivity.

[0080] 3. Reservoir space structure and diagenetic evolution path

[0081] Using CT scanning and argon-ion polishing-scanning electron microscopy, four types of reservoir spaces were identified in the limestone: intergranular pores, intragranular pores, organic matter pores, and microfractures. Among them, argillaceous microcrystalline limestone and argillaceous limestone, due to their higher clay mineral content, exhibited relatively larger pore volume and specific surface area. Correlation analysis showed that quartz, clay, and TOC were positively correlated with total pore volume and specific surface area, while calcite showed a negative correlation.

[0082] Based on homogenization temperature analysis of fluid inclusions in calcite veins, three phases of fluid activity were identified (temperature ranges of 65-110℃, 110-130℃, and 130-165℃, respectively). This was further analyzed using U-Pb dating and carbon-oxygen-strontium isotope (δ¹⁸O) dating techniques. 13 C、δ 18 O、 87 Sr / 86 Sr) constrains the diagenetic sequence and porosity formation stages. The Sr isotopic composition of the samples ranges from 0.707 to 0.708, with an average of 0.7079, consistent with contemporaneous seawater. This indicates that the limestone formed in a relatively stable marine sedimentary environment, during which rapid crystallization of carbonate minerals captured large amounts of seawater Sr. 87 Sr / 86 The Sr ratio was consistent with the seawater level at that time and was not significantly affected by land-based input or subsequent alteration.

[0083] By micro-sampling or separation of different diagenetic components, their calcium isotope composition (δ¹²) was analyzed. 44Ca), thus revealing its diagenetic alteration process: if a certain calcite vein δ 44 The abnormally positive Ca value suggests that it may have formed in a closed system dominated by atmospheric freshwater; if the δ of different pulse phases... 44 The Ca values ​​exhibit systematic variations, which, combined with Sr isotope analysis, can indicate the evolutionary path of diagenetic fluids from marine to terrestrial or deep-seated formation water. Bedrock and cement δ¹⁸O 44 Comparison of Ca values ​​can be used to assess the diagenetic alteration intensity of the original sediments. If the bedrock δ 44 The Ca values ​​differ significantly from marine standard values, indicating that the original sediments underwent intense diagenetic fluid alteration. In this study, δ 44 The systematic shift in Ca values ​​indicates that the sample underwent diagenetic alteration primarily through freshwater leaching and / or burial fluid exchange, but the original isotopic signals were not completely reset. This provides an important constraint for further quantifying the properties of diagenetic fluids and reconstructing diagenetic temperature and pressure conditions.

[0084] 4. Establishment of evaluation standards for high-quality reservoirs

[0085] Based on the above experimental data and geological modeling, the following evaluation criteria for high-quality lithofacies in the Lei 3 Member are established:

[0086]

[0087] This embodiment, through systematic experiments and multi-parameter fusion analysis, has achieved a complete characterization of the entire process of the three limestone reservoirs of the Leikoupo Formation, from microscopic pore structure to macroscopic diagenetic evolution. It provides a replicable and scalable technical path and evaluation standard for the exploration of similar ultra-deep marine carbonate reservoirs.

[0088] The above embodiments are merely one of the preferred embodiments of the present invention and should not be used to limit the scope of protection of the present invention. Any modifications or refinements made to the main design concept and spirit of the present invention that are not of substantial significance, but solve the same technical problem as the present invention, should be included within the scope of protection of the present invention.

Claims

1. A method for quantitatively analyzing the formation mechanism of ultra-deep limestone reservoirs, characterized in that, Includes the following steps: S1, core samples were selected from ultra-deep limestone reservoir sections for observation and sampling, and analytical samples were prepared; S2, mineral and microstructure analysis, organic geochemical analysis, major and trace element analysis, isotope analysis, and fluid inclusion analysis are performed on the sample to obtain multi-dimensional experimental data; wherein, the isotope analysis includes calcium isotope δ 44 / 40 Ca analysis; S3, based on experimental data, reconstructs the paleosedimentary environment, quantitatively characterizes the reservoir space features, and analyzes the properties and evolution history of diagenetic fluids; S4, the absolute age of the diagenetic event was determined by in-situ micro-area calcite U-Pb dating, combined with calcium isotope δ¹⁴ dating. 44 / 40 Ca data clarifies the formation period and preservation mechanism of reservoir porosity; S5, construct a quantitative diagenetic model of "fluid-time-reaction", establish a high-quality lithofacies evaluation standard, and complete the quantitative analysis of reservoir formation mechanism.

2. The method for quantitatively analyzing the formation mechanism of ultra-deep limestone reservoirs according to claim 1, characterized in that, In step S2, the mineral and microstructure analysis is performed using one or more of the following techniques: polarizing microscope, high-resolution scanning electron microscope, scanning electron microscope (SEM), X-ray diffraction (XRD), cathodoluminescence (CL), and CT scanning.

3. The method for quantitatively analyzing the formation mechanism of ultra-deep limestone reservoirs according to claim 2, characterized in that, The isotope analysis in step S2 also includes carbon and oxygen isotope analysis and strontium isotope analysis; the calcium isotope δ 44 / 40 Ca analysis was performed using a multi-receiver inductively coupled plasma mass spectrometer to trace the fluid source and the water-rock interaction process.

4. The method for quantitatively analyzing the formation mechanism of ultra-deep limestone reservoirs according to claim 3, characterized in that, The organic geochemical analysis described in step S2 includes TOC detection, saturated hydrocarbon chromatography-mass spectrometry analysis, and biomarker compound analysis to determine the abundance, type, maturity, and source of organic matter.

5. A method for quantitatively analyzing the formation mechanism of ultra-deep limestone reservoirs according to claim 1 or 4, characterized in that, In step S3, paleosalinity, redox environment and paleoproductivity are reconstructed using major and trace elements, rare earth elements and biomarker compounds, revealing the mechanism of organic matter enrichment.

6. The method for quantitatively analyzing the formation mechanism of ultra-deep limestone reservoirs according to claim 5, characterized in that, In step S4, by comparing the calcite U-Pb dating results with the formation time of the limestone, it is determined whether the pores were formed during the sedimentary or diagenetic period; and this is combined with the calcium isotope δ¹⁴Tb dating results. 44 / 40 By studying the variation characteristics of Ca data, we can identify the initial carbonate mineral transformation process and analyze the porosity preservation mechanism.

7. The method for quantitatively analyzing the formation mechanism of ultra-deep limestone reservoirs according to claim 6, characterized in that, The quantitative diagenetic model described in step S5 uses paleotemperature and paleopressure data measured by fluid inclusions, along with calcium isotope and rare earth element characteristics, to achieve quantitative characterization of the diagenetic evolution process.

8. The method for quantitatively analyzing the formation mechanism of ultra-deep limestone reservoirs according to claim 7, characterized in that, In step S5, the evaluation criteria for high-quality lithofacies include the abundance of organic matter, the maturity of organic matter, terrigenous input indicators, the degree of development of nanoscale pores in reservoir space, and geochemical indicators of diagenetic cementation strength.

9. The method for quantitatively analyzing the formation mechanism of ultra-deep limestone reservoirs according to claim 8, characterized in that, In step S2, the major and trace element analysis includes the determination of Sr, Mg, Fe, Mn and rare earth elements, the paleosalinity is determined by the Sr / Mg ratio, and the paleoredox conditions are inverted by the Fe / Mn ratio.

10. The method for quantitatively analyzing the formation mechanism of ultra-deep limestone reservoirs according to claim 9, characterized in that, In step S4, cathodoluminescence images are used to identify the calcite growth sequence, and combined with the differences in Fe and Mn content, cement formed in reducing burial environments and oxidizing near-surface diagenetic environments is distinguished.

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