Gaoshiti-Moxi Area Research Articles

Breakthrough and significance of technology on internal multiple recognition and suppression: A case study of Sinian Dengying Formation in Central Sichuan Basin, SW China

Abstract Aimed at the serious mismatch between the synthetic seismogram and the real data of the Sinian Dengying Formation and the Lower Cambrian Qiongzhusi Formation in the GS1 well area, Sichuan Basin, four aspects of internal multiples identification and suppression were studied. Firstly, a forward modeling method of internal multiple based on reflectivity method was developed. Through eight means such as post-stack and pre-stack forward modeling of internal multiple, and combined with VSP data, it was demonstrated that well-seismic mismatching in this area is mainly caused by the internal multiples. Secondly, the simulation results of internal multiple forward modeling using the stripping method combined with the internal multiple periodicity analysis showed that four groups of overburden layers with velocity inversion were the main sources of the internal multiples. Thirdly, by identifying internal multiples accurately and using suppression technology based on pattern recognition, an effective and replicable suppression scheme suitable for these formations was established, overcoming the difficulty of the small speed difference between internal multiple and primary reflection wave which makes the current methods ineffective. Fourthly, an evaluation index of internal multiple intensity was proposed, and the internal multiple intensity distribution diagram of the fourth member of Dengying Formation (Deng-4 Member) in Gaoshiti-Moxi area was compiled. This scheme greatly improved the well-seismic match, and the strata sedimentary features are clearer on the new seismic profiles with higher lateral resolution, with which smaller faults and geological anomalies can be identified and a series of the bead reflections in the Dengying Formation are first discovered. The coincidence rate of reservoir prediction based on seismic waveform classification was increased from 60% to 90%, and that of hydrocarbon detection based on dual phase medium theory was increased from 70% to 100%.

Strike-slip faults and their significance for hydrocarbon accumulation in Gaoshiti–Moxi area, Sichuan Basin, SW China

Abstract Based on the three dimensional seismic data and drilling data in Gaoshiti–Moxi area, the structural characteristics and evolution process of strike-slip faults in the study area and their significance for gas accumulation were examined using the method of fault structure analysis. The results show that: (1) Transtensional strike-slip faults are developed in the Paleozoic in study area. On the profile, there are three types of structural styles: steep and erect, flower structures, “Y” and reversed “Y” type faults. On the plane, strike-slip faults developed in the Cambrian extending linearly and along the nearly EW, NW and NE direction. The faults are composed of multiple secondary faults in en-echelon arrangement; faults in the Permian are mainly along nearly EW and NW direction, and the Permian faults are distributed in the middle east part of the study area and they are more in the north than in the south. (2) The nearly EW and NW trending faults are dextral strike-slip faults, and the NE trending faults are sinistral strike-slip faults. The strike-slip strength of the nearly EW trending faults are the strongest and the maximum horizontal displacement is 550 m, while the strike-slip strength of the NE trending faults is the weakest. The Cambrian faults had stronger activities than the Permian faults. (3) The transtensional strike-slip faults were active in two periods, the early Caledonian period and the late Hercynian period, and were the products of oblique extension of pre-existing weak zones in Xingkai and Emei taphrogenesis, with certain inheritance in the main faults. (4) The strike-slip faults and surrounding fractures enhance the porosity and permeability of the reservoir in Cambrian Longwangmiao Formation and control the distribution of karst reservoirs in the Permian Qixia Formation–Maokou Formation, bringing about the situation of multiple gases bearing series in this area.

Carbonate seismic sedimentology: A case study of Cambrian Longwangmiao Formation, Gaoshiti-Moxi area, Sichuan Basin, China

This study applied seismic-sedimentological workflow to deeply buried marine carbonate sequences in western China. The workflow aimed at integrating core, wire line log and 3D seismic data to investigate the paleogeomorphology qualitatively and reservoir thickness quantitatively in the carbonate systems. In Gaoshiti-Moxi area of Sichuan Basin, the Longwangmiao Formation was formed in sedimentary environment of carbonate platform facies. Lithofacies vary from dolomitized grainstone to mudstone, which formed shoal, lagoon, and deep shelf sub-facies. The high-porosity (porosity > 4%) reservoir rocks are related to shoal sub-facies. Seismic geomorphology was employed to restore paleostructure of Longwangmiao Formation in the study area by methods of cast and isopach, which were calibrated by well data. It was found that the paleogeomorphology and depositional facies were largely controlled by a syndepositional, en echelon fault system. Following the strategy of seismic lithology, quantitative distribution of reservoir rocks and sedimentary facies was assessed by applying principle component analysis on amplitude attributes generated from multiple frequency panels. Seismic prediction of diagenetic facies (selective dissolution) was achieved by interpreting similarity variance, a seismic geometric attribute. In summary, the procedure, combining seismic geomorphology and seismic lithology, is effective and can be used as a reference for seismic-sedimentological study in other carbonate basins in China and elsewhere.

Precambrian temperature and pressure system of Gaoshiti-Moxi block in the central paleo-uplift of Sichuan Basin, southwest China

Recently, trillions of cubic meters gas resources have been found in Sinian reservoirs of Gaoshiti-Moxi region in the central uplift of Sichuan Basin, which is the first commercial discovery of primary gas field in Precambrian anywhere in the world. To obtain a great breakthrough in Precambrian hydrocarbon exploration, it is necessary to find out the evolution process of temperature and pressure. Reconstructing the thermal history and pore pressure evolution in carbonate reservoirs are challenging problems, especially in deep-old layers without available samples and methods. To study the temperature and pressure system, we conducted a comprehensive analysis combining the paleo-thermal indicators and inclusions Pressure-Volume-Temperature (PVT) simulation with basin modeling. The results showed that the thermal history of the Gaoshiti-Moxi area could be divided into three stages. From the Late Sinian to Late Paleozoic, the thermal regime was stable with low heat flow value (<65 mW/m2). During the Early Paleozoic, the heat flow increased rapidly to the peak value (75–100 mW/m2). From Mesozoic to present, the thermal subsidence occurred in the basin with the heat flow decreasing to the present value (60–70 mW/m2). With the boundary condition of above thermal history, the simulated pressure of Dengying Formation in Gaoshiti-Moxi area could be divided into four stages. The excess pressure began to form during the Triassic. From the end of Triassic to Middle Jurassic, the excess pressure increased steadily and slowly. From Middle Jurassic to the end of Early Cretaceous, the strata subsided rapidly while the excess pressure increased significantly. Since the Late Cretaceous, the Dengying Formation suffered from rapid uplifting and gas lateral migrating with the excess pressure decreasing, until the Late Neogene, the excess pressure in reservoirs declined to zero.

The evolution of pore-fluid pressure and its causes in the Sinian-Cambrian deep carbonate gas reservoirs in central Sichuan Basin, southwestern China

Reconstructing the evolution of paleofluid pressure in carbonate reservoirs is a challenging problem, particularly when no oil-bearing fluid inclusions are available to provide barometric constraints on the fluid system. To recover the paleofluid pressure in the Sinian Dengying Formation (Z2dn) and the Cambrian Longwangmiao Formation (Є1l) of the central paleo-uplift of the Sichuan Basin, we conducted a comprehensive analysis combining the inclusion pressure-volume-temperature simulation, inclusions in situ Raman shifts, and basin modeling. The results showed that the evolution of the pore pressure could be divided into four stages. Prior to the end of the late Triassic, the whole central paleo-uplift was in a normal pressure state. During the Jurassic, a moderate overpressure prevailed in the dolomitic reservoirs of the Gaoshiti-Moxi area and decreased in the adjacent units. At the end of the early Cretaceous, the overpressure reached a maximum in the whole region and was strongest in the Longnüsi area, decreasing to the west. From the late Cretaceous to the present day, the central area of the basin has uplifted rapidly with the overpressure significantly decreasing. Z2dn has been restored to normal pressure during the Neogene, while Є1l remains moderately overpressurized. Based on an analysis of the structural background and preservation conditions, the types of sedimentary facies, the preservation condition of the traps, and the differential weathering process are the primary reasons for the different pressure states in Z2dn and Є1l during the last stages.

The pre-Sinian rift in central Sichuan Basin and its control on hydrocarbon accumulation in the overlying strata

Abstract Based on the latest seismic data, resistivity profile, outcrop evidence and logging data, the structural features of basement in Sichuan Basin and its control on the hydrocarbon accumulation in the Sinian-Cambrian strata was discussed. It was found that a NE striking pre-Sinian rift was developed across the whole basin. Controlled by a series of rift-parallel normal faults, horst-graben structures were developed inside the rift, large horst-graben structures and later activity of their boundary faults controlled the distribution of beach facies of the overlying strata. The horst-graben structures induced the formation of local highs of ancient landform and controlled the successive development of overlapped bioherm beach facies in long-term marine setting from the Sinian period to the Permian period, and as a result a widely distributed favorable sedimentary facies belt was developed. The pre-Sinian rift and later activities of related normal faults controlled the development of the grain beach and karst reservoirs and the deposition of high quality source rock, which form structural-lithologic traps. Through comprehensive evaluation, two large structural-lithologic composite trap favorable exploration areas in the south and north of the Gaoshiti-Moxi area, were selected.

Challenges and Methods to Improve Well Cementing Quality for 177.8 mm Liner in Gaoshiti-Moxi Area

Gaoshiti-Moxi structure belt of An-Yue Sinian gas reservoir, which was China’s largest monomer Marine carbonate gas reservoir up to now, located in vying-dragon female temple structure group that belong to the ancient uplift slope of the middle of Sichuan. With the exploration and development of high temperature and high pressure carbonate reservoir, a large number of challenges and problems, such as long isolation section, active oil-gas show, large temperature difference, prone to super retarding cement slurry and gas channeling at flare position, have been encountered in the cementing of 177.8 mm hang-liner. In order to solve these problems, numerous measures and methods have been put into use for reducing the safety risk of cementing and improving cementing quality. The large temperature difference channeling cement slurry system, effective anti-pollution spacer and high-pressure packer type liner hanger were developed and applied for field tests in the early stage of development. In addition, equilibrium pressure cementing technology, optimizing of centralizer placement and plasma column structure, improvement of pump displacement and hold pressure while waiting on cement were used to ensure nice displacement efficiency and cementing quality. As Moxi X well for example, the cementing quality factor of merit was 40.29% and the qualification rate was up to 78.87% after adopting the cementing measures and methods above. The cementing quality was much higher than previous level and provided technical support for Gaoshiti-Moxi area.

Seismic prediction on the favorable efficient development areas of the Longwangmiao Fm gas reservoir in the Gaoshiti–Moxi area, Sichuan Basin

The Lower Cambrian Longwangmiao Fm gas reservoir in the Gaoshiti–Moxi area, the Sichuan Basin, is a super giant monoblock marine carbonate gas reservoir with its single size being the largest in China. The key to the realization of high and stable production gas wells in this gas reservoir is to identify accurately high-permeability zones where there are dissolved pores or dissolved pores are superimposed with fractures. However, high quality dolomite reservoirs are characterized by large burial depth and strong heterogeneity, so reservoir prediction is of difficult. In this paper, related seismic researches were carried out and supporting technologies were developed as follows. First, a geologic model was built after an analysis of the existing data and forward modeling was carried out to establish a reservoir seismic response model. Second, by virtue of well-oriented amplitude processing technology, spherical diffusion compensation factor was obtained based on VSP well logging data and the true amplitude of seismic data was recovered. Third, the resolution of deep seismic data was improved by using the well-oriented high-resolution frequency-expanding technology and prestack time migration data of high quality was acquired. And fourth, multiple shoal facies reservoirs were traced by using the global automatic seismic interpretation technology which is based on stratigraphic model, multiple reservoirs which are laterally continuous and vertically superimposed could be predicted, and the areal distribution of high quality reservoirs could be described accurately and efficiently. By virtue of the supporting technologies, drilling trajectory is positioned accurately, and the deployed development wells all have high yield. These technologies also promote the construction of a modern supergiant gas field of tens of billions of cubic meters.

Controlling factors of Dengying Formation reservoirs in the central Sichuan paleo-uplift

Macroscopic and microscopic characteristics revealed that the Dengying Formation reservoir in the central Sichuan paleo-uplift belonged to the compound genetic reservoir of the mound-shoal facies and karst. Development of the reservoir was controlled by the mound-shoal facies, and dissolution-enlarged or added pores (vugs) due to superimposed karstification modification were favorable reservoir spaces. The karst of the Dengying Formation in Tongwan stage was the stratabound-weathered crust karst of the early diagenesis stage, the extensive superimposition of mound-shoal facies and regional karstification in the central Sichuan paleo-uplift area was the key to develop the large-scale continuous Dengying Formation reservoirs. Due to differences in the development degree of mound-shoal facies and in the intensity of karst modification, the reservoir development degree varied in different zones in the paleo-uplift area. The NS-extending zone on the western side of the Gaoshiti-Moxi area was the sweet spot for reservoir development. Effective identification of mound-shoal facies and fine depiction of karst landform was the key for reservoir prediction of the Dengying Formation to discovery the superimposed area of the mound-shoal facies and the karst monadnock-karst steep slope which similar to Moxi-Gaoshiti area.

Origin and Distribution of Grain Dolostone Reservoirs in the Cambrian Longwangmiao Formation, Sichuan Basin, China

Dolostones in the Cambrian Longwangmiao Formation have become one of the most significant gas exploration domains in China. Over a trillion cubic meters of gas reservoirs have been discovered in the Gaoshiti-Moxi area; however, the origins and distribution of the dolostone reservoirs are not well understood. This work discussed the geology and geochemistry of the dolostone reservoirs in the Longwangmiao Formation to determine their origin and distribution. Two understandings are acquired: firstly, a carbonate ramp provided excellent conditions for grain beach deposition, while the presence of a hypersaline lake was favorable for the contemporaneous dolomitization of grain beach deposits. Petrographic and geochemical evidence further confirm that the Longwangmiao dolostone was formed during the contemporaneous stage. Secondly, the reservoir characteristics indicate that the grain beach sediments provide material basis for the development of the Longwangmiao dolostone reservoirs. Reservoir dissolution simulation experiments show that the porosity of the reservoirs was formed by dissolution during contemporaneous and burial stages. The dissolution pores formed during the contemporaneous stage were controlled by sequence interfaces. The large scale dissolution vugs formed during the burial stage subsequently spread along the pre-existing porosity and fracture zones. This study therefore identified that the development of grain dolostone reservoirs in a shallow water ramp under arid climatic conditions generally met the following conditions: (1) reefal beach deposits lay a foundation for reservoir development; (2) superficial conditions are an important determining factor for reservoir porosity; and (3) burial conditions provide environment for porosity preservation and modification.

Karst paleogeomorphology of the fourth Member of Sinian Dengying Formation in Gaoshiti-Moxi area, Sichuan Basin, SW China: Restoration and geological significance

Abstract Based on 3-D seismic data and drill-hole data in Moxi-Gaoshiti area, the impression seismic thickness from the top of Longwangmiao Formation, which is easier traced, to the bottom of Cambrian System was selected to characterize the karst paleogeomorphology of the fourth Member of Dengying Formation. The paleogeomorphology of the member can be further divided into three geomorphic units: karst platform, slope and superimposed slope, which had different paleohydrologic conditions, and thus different karstification intensity and reservoir quality. Among them, the superimposed slope with the strongest dissolution, has larger solution groove and cave systems, where the reservoirs with mainly honeycombed dissolved pores and cavities as storage space, are best in quality; the platform slope with weaker dissolution, has smaller solution groove and cave systems, where the reservoirs are poorer than those in the superimposed slope; the karst platform with the weakest dissolution, has piebald karst systems mainly, where the reservoirs with mainly pinhole to smaller dissolved pores and cavities as storage space, are poorer in quality on the whole, but there are some good quality reservoir bodies in local areas. The results show that, besides the highly explored scarp belt, the slope in the intra-platform with low exploration degree can also be favorable exploration area of the fourth member of Dengying Formation.

Characteristics of Dengying Fm sedimentary sequence in the central–western Sichuan Basin and their controlling effect on gas accumulation

A large gas reservoir in the Sinian Dengying Fm was recently discovered in Anyue area, central Sichuan Basin. In order to promote the oil and gas exploration in this area, it is of great significance to sustainably deepen the analysis on its high-quality reservoir distribution and source-reservoir assemblage. In this paper, a series of studies were performed on the sequence frameworks and sedimentary filling evolution characteristics of the Dengying Fm in central–western Sichuan Basin, and their controlling effects on the reservoir development and distribution and source-reservoir assemblage during its sedimentation. It is shown that the Deyang–Anyue paleo-rift is, on the whole, distributed in a NW direction, and is morphologically wide and steep in the north, but narrow and gentle in the south. There are two rift boundaries of the second and fourth members of Dengying Fm (Deng 2 Member and Deng 4 Member, respectively). The rift of Deng 2 Member is distributed in the shape of “U” in Deyang–Neijiang area. Dengying Fm is divided into four third-order sequences (SQ1–SQ4). In SQ3 of Gaoshiti–Moxi area, central Sichuan Basin, a complete sedimentation reflection structure of transgressive overlap and regressive progradation can be identified with early–middle Deng 4 Member platform margin. The paleo-rift was in embryo before the sedimentation of Sinian, so the sedimentary filling and lithofacies distribution of the Sinian were influenced by the inherited activities of pre-Sinian rifts. The favorable distribution zones of high-quality reservoirs are located at the rift margins of Deng 2 and Deng 4 Members that were formed respectively during the SQ2 and SQ4 and the platform margin of early Deng 4 Member that was formed during the SQ3. A laterally connected source-reservoir assemblage occurs at the margins of Deng 2 and Deng 4 Members around the Lower Cambrian source rocks of the paleo-rift. The northwestward extending margin zone along the eastern flank of the paleo-rift shows a good potential for hydrocarbon accumulation.

Key seismic exploration technology for the Longwangmiao Fm gas reservoir in Gaoshiti–Moxi area, Sichuan Basin

The dolomite reservoirs of the Lower Cambrian Longwangmiao Fm in the Gaoshiti–Moxi area, Sichuan Basin, are deeply buried (generally 4400–4900 m), with high heterogeneity, making reservoir prediction difficult. In this regard, key seismic exploration technologies were developed through researches. Firstly, through in-depth analysis on the existing geologic, drilling, seismic data and available research findings, basic surface and subsurface structures and geologic conditions within the study area were clarified. Secondly, digital seismic data acquisition technologies with wide azimuth, wide frequency band and minor bins were adopted to ensure even distribution of coverage of target formations through optimization of the 3D seismic geometry. In this way, high-accuracy 3D seismic data can be acquired through shallow, middle and deep formations. Thirdly, well-control seismic data processing technologies were applied to enhance the signal-to-noise ratio (SNR) of seismic data for deep formations. Fourthly, a seismic response model was established specifically for the Longwangmiao Fm reservoir. Quantitative prediction of the reservoir was performed through pre-stack geo-statistics. In this way, plan distribution of reservoir thicknesses was mapped. Fifthly, core tests and logging data analysis were conducted to determine gas-sensitive elastic parameters, which were then used in pre-stack hydrocarbon detection to eliminate the multiple solutions in seismic data interpretation. It is concluded that application of the above-mentioned key technologies effectively promote the discovery of largescale marine carbonate gas reservoirs of the Longwangmiao Fm.

The influences of sea-level changes on the quality of bank reservoirs of the Lower Cambrian Longwangmiao Formation, in the Gaoshiti-Moxi area, Sichuan Province, China

Reservoir properties of banks of the Longwangmiao Formation show strong lateral and vertical variations in the Gaoshiti-Moxi area, central Sichuan. In this study, we utilized outcrop, core data, thin sections, wireline logs, and seismic data to investigate the influences of sea-level changes on the development of banks and penecontemporaneous dissolution which controlled the formation of high-quality bank reservoirs. Our results revealed that the Longwangmiao Formation corresponds to a third-order sea-level change cycle, which consisted of a rapid sea-level rise and a slower sea-level fall; we further identify four fourth-order sea-level cycles. Bank reservoirs are dominated by grain dolostone, silt to fine sand-sized crystalline dolostone, and mottled silt to fine sand-sized crystalline dolostone. The main reservoir spaces were pores and vugs, with an average porosity of 4.81% and average permeability of 4.75 × 10−3 μm2. Bank deposition and dissolution during the third-order sea-level fall developed better than during the rising period, and the late fall stage was better than the early fall stage. Under the influence of fourth-order sea-level changes within the third-order sea-level change, the development of banks shows strong variations between different areas, which are evidenced by the fact that banks in the Moxi block were better developed than those in the Gaoshiti block. Bank dissolution in the Moxi block developed during the early and the late stage within the third-order sea-level fall, whereas the Gaoshiti block was developed only during the late stage. Therefore, we have concluded that sea-level changes played a significant role in controlling the quality of bank reservoirs in the Gaoshiti-Moxi area.

Hydrothermal dolomitization in Dengying Formation, Gaoshiti-Moxi area, Sichuan Basin, SW China

Abstract The formation conditions, existence evidence, modification modes and timing of hydrothermal dolomitization in the Sinian Dengying Formation are studied based on the basic geological conditions, mineral assemblages in dolomites, and geochemical features of the Gaoshiti-Moxi area in the Sichuan Basin. The Gaoshiti-Moxi area is a significant exploration target of the Sinian Dengying Formation in the Sichuan Basin and has the basic conditions of regional geology for the occurrence of structurally controlled hydrothermal dolomitization: (1) activity of extensional basement-fault; (2) deep-burial hydrothermal reservoirs; (3) the overlying seal strata. Based on the petrographic analysis and geochemical tests (trace elements (Fe and Mn), stable isotopes (C, O, and Sr), homogenization temperature of fluid inclusions, etc.) of the core samples of the Dengying Formation in the study area, combing with the MVT mineral assemblages and geochemical characteristics, the study demonstrates that structurally controlled hydrothermal dolomitization exists in the Dengying Formation of the study area. This type of hydrothermal dolomitization refers to transformation of matrix dolomites by hydrothermal fluids and it consists of three modes: (1) dissolution and cementation; (2) recrystallization and neomorphism; (3) hydrofracturing. It is inferred that there exists multi period of hydrothermal dolomitization, namely, Late Sinian to Early Cambrian, Late Devonian and Late Permian.

Features and origin of natural gas in the Sinian–Cambrian of central Sichuan paleo-uplift, Sichuan Basin, SW China

Abstract Based on the new drilling data and field outcrop data of the Gaoshiti–Moxi area, the geochemical characteristics of the Sinian-Cambrian natural gas are studied and analyzed, including gas composition, isotope, light hydrocarbon, kerogen carbon isotope and reservoir bitumen biomarkers etc. The results show that: (1) The natural gases of the Sinian Dengying Formation and Cambrian Longwangmiao Formation, mainly composed of hydrocarbon gas, are typical dry gas. However, the natural gas of the Dengying Formation is characterized by higher dry coefficient, lower content of hydrocarbon gas and higher content of non-hydrocarbon gas. The main differences in non-hydrocarbon gases are the contents of N 2 , CO 2 , H 2 S and He, the small composition differences between the natural gas of Dengying Formation and that of Longwangmiao Formation are mainly caused by maturity differences of source rocks and H 2 S generated by reaction between sulfide mineral and hydrocarbons. (2) There are obvious differences in δ 13 C 2 between the natural gases in Dengying Formation and Longwangmiao Formation, showing different parent materials of them. (3) There are large differences in δ 2 H between the natural gases of Dengying Formation and Longwangmiao Formation, showing the different water salinities of their source rock depositional environment. (4) The average kerogen carbon isotope values of the Lower Cambrian shale, Dengying Formation mudstone, Doushantuo Formation mudstone and Dengying Formation carbonates are −32.8‰, −31.9‰, −30.7‰ and −27.8‰ respectively. (5) The ratio of 4-methyl dibenzothiophene to 1-methyl dibenzothiophene of Dengying Formation bitumen is between that of Qiongzhusi Formation and Dengying Formation source rocks. It is believed that the Sinian-Cambrian natural gas in the Gaoshiti-Moxi area is mainly oil cracking gas, the Sinian natural gases come from the Sinian and Cambrian source rocks, and the Cambrian natural gases mainly come from Cambrian source rock.

Characteristics and Enrichment Factors of Supergiant Lower Cambrian Longwangmiao Gas Reservoir in Anyue Gas Field: The Oldest and Largest Single Monoblock Gas Reservoir in China

An ultra-deep monoblock marine carbonate gas reservoir with the highest maturity and largest individual scale in China has been discovered in Lower Cambrian Longwangmiao Formation (LF) of Anyue gas field in the last few years. Based on a thorough study on Longwangmiao gas reservoir in the Gaoshiti-Moxi(GM) area, this paper presents the characteristics of such formation in Anyue gas field as follows: ① it is a lithologic-anticlinal gas reservoir developed on a gentle paleo-uplift; ② the gas reservoir is characterized by high temperature and pressure, contains sulfur, and belongs to a dry gas reservoir; and ③ the upper part of the gas reservoir has a good connectivity along with a free of bottom water and edge water, but poor in the lower part, where small scope of enclosed CaCl2 water exists locally. This paper further studies the main control factors for forming this supergiant gas reservoir: ① synsedimentary paleo-uplift controlled the distribution of large-area particle shoal, superimposed by multiple corrosion, which high quality reservoir was formed; ② syngenetic extended aulacogen was filled by mudstone and shale rapidly, resulted in widely distributed thick source rock; ③ synsedimentary gypsum rock and densified carbonate rock formed lateral sealing, multiple type of cap rocks superimposed vertically, therefore excellent closure conditions were formed; ④ large scale of gas reservoir trap was formed at early stage, and it was in a favorable hydrocarbon accumulation pointing area for a long time; and ⑤ the effective time-space matching of numerous hydrocarbon accumulation elements is the key to the formation of the supergiant gas reservoir. The understanding on the formation mechanism and enrichment rule of this ultra-deep, high temperature and high pressure gas reservoir will theoretically enrich the petroleum geology theory, and practically bring important reference to the exploration of large gas fields in the deep ancient craton basins with high maturity and multiple tectonic evolutions in the world.

Tectonic features of Gaoshiti-Moxi paleo-uplift and its controls on the formation of a giant gas field, Sichuan Basin, SW China

Abstract Based on the latest geological, seismic, drilling and outcrop data in the Sichuan Basin, the geological structure and evolution history of Gaoshiti-Moxi paleo-uplift was studied to find out controlling factors of the Sinian-Cambrian Anyue giant gas field. The Gaoshiti-Moxi paleo-uplift is an N-S trending syndepositional paleo-uplift related to the Tongwan movement. The top of Sinian Dengying Formation and adjacent strata in the central part of the paleo-uplift, namely the Gaoshiti-Moxi Area, has remained in the relatively high position since the Sinian, and a giant trap structure is developed independently in the area, which is different from Caledonian Leshan-Longnusi paleo-uplift in development time, geological structure and evolution history. The Gaoshiti-Moxi paleo-uplift controls the development and distribution of the Sinian-Cambrian Anyue giant gas field, which is shown in the following aspects: (1) The western part of the paleo-uplift is adjacent to the source rock center of Cambrian Maidiping and Qiongzhusi Formations in Mianzhu-Changning intracratonic taphrogenic trough, and the source rocks of Sinian Dengying Formation and Cambrian Qiongzhusi Formation also occur in the Gaoshiti-Moxi paleo-uplift itself; (2) the paleo-uplift controls the formation and distribution of the high-quality reservoirs of the fourth and second members of Sinian Dengying Formation and Cambrian Longwangmiao Formation; and (3) there develop 3 sets of reservoir-seal assemblages which provide favorable conditions for the formation of the giant gas pool from in-situ cracking of paleo-oil reservoirs in the Sinian-Cambrian.

Geological features of Sinian-Early Cambrian intracratonic rift of the Sichuan Basin

With the major breakthrough in Sinian-Cambrian gas exploration in the Sichuan Basin, the geologic understanding on this formation has changed. One change concerns the discovery of a nearly SN negative structure in the west of Gaoshiti-Moxi area, but there is controversy over the nature, boundary features and distribution scope, sedimentary features, formation stage and evolution of the structure [11–14]. Since the negative structure belt is closely related to the formation of the Anyue Sinian-Cambrian giant gas field, it is necessary to get a better understanding of its geological features in order to guide the gas exploration in Sinian-Cambrian of the Sichuan Basin in the future. Through an analysis of the drilling data of Sinian-Cambrian in this area, an observation of outcrop profiles and the use of 27000 km 2D seismic data and latest drilling data (Well Gaoshi 17), and in combination with regional structure background, the boundary and distribution scope, sedimentary features and evolution of the Mianzhu-Changning rift in craton were investigated. The results showed that: ① the rift in overall SN direction had a steep and fairly stable eastern boundary, and gentler western boundary with variable positions in different stages; ② at the end of Sinian Deng2 member, the rift was a nearly symmetrical depression with an area of 3 × 104 km2; ③ at the end of Deng4 member, it was a fault depression which was steep in the east and gentle in the west controlled by its eastern boundary fault with an area of 8 × 104 km2; and ④ the evolution of the rift can be divided into four stages: embryonic stage in Deng1-Deng2, formation stage in Deng3-Deng4, filling and subsidence stage in Early Cambrian Maidingping-Qiongzhusi Formation, shrinkage and disappearing stage in Early Cambrian Canglangpu-Longwangmiao Formation. Through the study, it was concluded that both sides of the rift are in good Sinian-Cambrian gas-reservoir-forming conditions, which are major prospects for future exploration.

Geochemical characteristics and origin of Sinian-Lower Paleozoic natural gas in the Sichuan Basin

The composition and isotopic values of natural gas in the Sinian Dengying Fm, Cambrian Qiongzhusi Fm, Longwangmiao Fm and Xixiangchi Fm in the Sichuan Basin exhibit different features in different tectonic position, causing controversy over its origin and sources. The geochemical features of the natural gases in this area were compared based on previous research results and large amounts of drilling data of new exploration wells. The results show: (1) The natural gas is typical dry gas on the whole, with hydrocarbon gas in dominance, and a methane content of 74.85%–97.35%, mostly 83.0%–96.0%; due to different clay contents in source rocks, the difference in non-hydrocarbon gas mainly lies in N2 and He content, with gas in Weiyuan and Ziyang areas having high contents of N2 and He, while gas in Gaoshiti and Moxi areas having low N2 and He. (2) Natural gases in different regions differ greatly in carbon isotope value (δ13C1 and δ13C2). The δ13C1 of natural gas in Cambrian system of Ziyang area is the lightest, from −38.0‰ to −35.5‰, and that in other regions is from −33.9‰ to −32.0‰, indicating different capture stages of natural gases. For example, gas captured in early stage is relatively light in carbon isotope; the carbon isotope value (δ13C2) of gas from Sinian and Cambrian system in Weiyuan is from −36.5‰ to −32.7‰; that from the Longwangmiao Fm in Gaoshiti and Moxi is from −33.6‰ to −31.8‰, but that in the Dengying Fm in Gaoshiti-Moxi area is from −29.1‰ to −26.8‰, quite different from the above natural gases, which mainly shows the differences in kerogen types. (3) Light hydrocarbons of C6–C7 mainly consist of cyclanes and isomerization alkanes, which is the feature of oil cracking gas.