Deeply buried dolomite reservoirs within craton basins are important hydrocarbon reservoirs. This study employs a comprehensive statistical analysis of key craton basins, including Sichuan and Tarim Basin. The analysis identifies three main types of deeply buried dolostone: granular dolostone, microbial dolostone and crystalline dolostone. These types are characterised by distinct pore structures, specifically cavity-type and pore-type. The study highlights the critical importance of favourable sedimentation conditions as the foundation for pore development, while also emphasising the essential role of early dissolution processes. Additionally, early dolomite replacement facilitates the preservation of primary porosity. Moreover, reservoir properties receive enhancement from epigenetic karstification and tectonic fracturing mechanisms. Notably, this study elucidates that deep-buried, high-quality dolomite reservoirs, of which storage and flow properties that are useful for hydrocarbon recovery, maintain a distinct level of stratigraphic control, with inheritance outweighing modification. Vertical distribution of the dolomite reservoirs is controlled by high-frequency cycles and the burial depth, and exhibiting periodic cycles, not entirely controlled by reservoirs burial depth, whereas plane distribution is ascribed to advantageous sedimentation instead of karstification processes. As the paper concludes, it advocates future exploration endeavours targeting the inner rift within the platform and carbonate ramp areas, thus illuminating promising directions to seek deep-buried effective dolostone reservoirs. The underground storage space of ultra-deep carbonate reservoirs is enormous, and it is currently the most feasible large-scale CCUS-EOR approach.

The intraplatform shoal dolomite of the Middle Permian Qixia Formation is currently considered the key target of hydrocarbon exploration in the central Sichuan Basin. To systematically investigate the origin of the stratabound facies-controlled porous dolomites of the Qixia Formation, integrated petrography, logging and seismic analysis were carried out in this work. The results are as following: (1) the dolomite reservoir is universal in the central Sichuan Basin, and its distribution is controlled by intraplatform shoals, with multilayer superposition vertically. Thick massive dolostone may also develop along with the fault. (2) Three replaced dolomites and one dolomite cement were identified: very finely to finely crystalline, anhedral to subhedral dolomite (Rd1); finely to medium crystalline, anhedral to subhedral dolomite (Rd2); coarsely crystalline, subhedral to euhedral dolomite (Rd3) and coarsely crystalline saddle dolomite cement (Sd). Rd2 and Rd3 are partly fabric-retentive, and preserve the original bioclastic ghosts. Sd shows wavy extinction, filled in the breccia veins. (3) The U-Pb dating and homogenization temperatures results indicate that the dolomite and Sd cement are associated with hydrothermal event during the Emeishan large igneous province. The δ13C, 87Sr/86Sr, and seawater-like REEY patterns suggest that the dolomitization and Sd precipitation fluids originate from connate seawater heated by elevated heat-flow. (4) The ELIP triggered large scale thermal anomalies in the basin during the Dongwu movement period. The increased temperature and pressure drove the formation water in the intra-platform shoal facies and overcame the binding effect of Mg2+ hydrate. Moreover, the deep hydrothermal fluid preferentially penetrated into the porous strata of shoal facies along the faults and fractures, mixed with formation water to some extent, and extensive dolomitization occurred. The facies-controlled dolomite reservoir and the underlying Cambrian source rock form a good source-reservoir assemblage, which can be a key replacement option.

To figure out gas enrichment laws in Permian volcanic rocks, Sichuan Basin, an association among reservoir-forming elements was explored to analyze forming modes and point out exploration prospects in accordance with reservoir characteristics, gas source conditions, and fault elongation in Zhougongshan and Jianyang areas. Results show that there mostly exist porous volcaniclastic rock and lava reservoirs of explosive facies. In Jianyang with Lower Cambrian Qiongzhusi Formation as the principal source rock, early faults offered pathways to assemble ample initial oil and gas. Following an increase of burial depth, oil started cracking vastly to create oil-cracked gas. These elements constitute one reservoir-forming mode of Cambrian hydrocarbon supply–fault transporting–in-situ cracking in Jianyang. While in Zhougongshan with Lower Permian Maokou 1 Member as the main source, active faults during hydrocarbon generation and expulsion acted on trap preservation. Late kerogen-cracked gas gathered at structural highs far from giant faults. Thus, the other mode exhibited in Zhougongshan is Permian hydrocarbon supply–fault adjustment–effective trap accumulation. Reservoir-forming conditions in volcanic rocks present certain disparity among all kinds of tectonic settings inside the basin, giving rise to various volcanic gas reservoirs developed. The Permian volcanic rocks are expected to be a major domain for reserve and production increase in Sichuan Basin.

Deep to ultra-deep carbonate rocks have become a target for hydrocarbon exploration. The formation mechanisms of these deep to ultra-deep reservoirs are very complicated, and no consensus has yet emerged. This paper reports an investigation into the Middle Devonian Guanwushan Formation in the NW Sichuan Basin, China, which is a typical ultra-deep dolomite reservoir. Drilling-core and thin-section observations identified breccia matrix dolomite (MD) and interbreccia-filling dolomite (FD), of which the FD is commonly associated with sphalerite and barite. Geochemical analyses, including C and O isotopes, trace and rare earth element (REE) contents, and fluid inclusions, were conducted to investigate the origins of the different types of dolomite. Results indicate that the MDs generally retain the signature of syn-depositional seawater (relatively positive δ18O, low Fe and Mn, left-leaning REE patterns, and negative Ce anomalies), with involvement of meteoric water (relatively high Al, Ti and ΣREE). In contrast, the FDs were the diagenetic product of deep hydrothermal fluids (depleted δ18O, high Fe and Mn, and enrichment in MREEs), and the homogenization temperature of fluid inclusions is much higher than the normal burial geothermal temperature. The breccia dolomite reservoirs of the Guanwushan Formation were deposited as reef-core micro-facies and formed during the depositional to early diagenetic stages, and were the result of subaerial seawater dolomitization and meteoric water karstification. However, hydrothermal fluids subsequently modified the reservoir by cementation and filling. The Guanwushan Formation is a typical case of an ultra-deep carbonate reservoir that formed during the depositional to early diagenetic stages but was filled at the burial stage. Therefore, the favorable reservoir zones associated with deposition and early diagenesis should be the targets of future exploration. The findings of this study are expected to be of general significance for deep and ultra-deep oil and gas exploration in other regions and countries.

The first member of Middle Permian Maokou Formation in Sichuan Basin was always regarded as a set of carbonate source rocks not as reservoirs for evaluation in previous exploration practices.So,XT1well,one risk exploration well,was taken as an objective to deeply figure out the natural-gas exploration potential of the first Member of Maokou Formation in eastern basin.Moreover,both mineral petrology and geochemistry were explored on the basis of core data through some methods,like physical property testing,thin section identification,and scanning electron microscope and organic geochemical analysis.Finally,characteristics as both reservoir rocks and source rocks were gained.Results show that(i)this member in eastern Sichuan Basin is dominated by medium or thin-medium layered dark-gray to gray-black argillaceous micritic bioclastic limestone and marlstone,and medium-thick to thick layered gray to dark-gray micritic bioclastic limestone and argillaceous micritic bioclastic limestone.Its depositional waterbody is relatively deep.A kind of"eyeball structure",which is formed by the interbedded micritic bioclastic limestone,argillaceous micritic bioclastic limestone and marlstone,is developed;(ii)for core plugs,their helium porosity ranges from 0.24%to 6.98%,1.39%on average,and the permeability from 0.0014 mD to 1.72 mD,0.083 mD on average,respectively,belonging to low-porosity and low-permeability reservoirs.Among which,the eyelid-shaped limestone boasts better physical properties,for example the porosity between 0.24%and 6.98%,and the reservoir space mainly consisting of sheet-like talc intercrystalline micropores and microfractures.While the eyeball limestone possesses poor storage performance with the porosity from 0.24%to 3.41%,resulting that this sort of reservoir rocks is not developed;(iii)with TOC between 0.17%and 1.63%,the eyelid limestone in this well enjoys certain hydrocarbon-generating ability.The source rocks of first Member of Maokou Formation in eastern basin are generally thicker than 80 m,thus suggesting good hydrocarbon-generating conditions;(iv)in this area,it can develop into argillaceous limestone and marlstone gas reservoirs with self-generation and self-storage as well as multiple source rocks with the favorable exploration area of 2×104 km2 and the pre-dicted resources over 2,000×108 m3,displaying large exploration prospects.

Based on analysis of field survey, drilling and seismic data, the formation and evolution process of Deyang-Anyue erosion rift trough in Sichuan Basin was reconstructed, and exploration areas were divided and evaluated. The results show that: (1) Dengying Formation in and around Deyang-Anyue erosion rift trough varies widely in sedimentary characteristics. The Dengying Formation in the northern part of the erosion rift trough developed deep-water sediments, the Dengying Formation in the northern part of the basin varied gradually from basin to slope, platform margin, and restricted platform, and the Dengying Formation in the middle and southern parts of the trough developed carbonate platform facies. (2) Deyang-Anyue erosion rift trough is formed by extensional rift and karst erosion jointly, the north section of the erosion rift trough is mainly the product of tensile rift, while the middle and south sections are formed by erosion in multi-episodes of Tongwan period. (3) Based on the segmented origins of the erosion rift trough, Dengying Formation in and around it is divided into three exploration fields: lithologic mound and beach bodies at the northern platform margin of the basin, karst mound and beach bodies in the central platform, and karst residual mounds in the central southern trough of the basin, among them, the karst residual mounds in the central southern trough of the basin are a new frontier for natural gas exploration in the basin, and the lithologic mound and beach bodies at the northern platform margin are a new position for increasing the reserves of trillions of cubic meters of natural gas resources in the basin.

震旦系灯影组四段是四川盆地安岳气田主力勘探产层之一,是一套受丘滩体发育和桐湾II幕表生岩溶作用共同控制的储层.灯四段储层纵横向非均质性强,相较于台缘带灯四段孔洞型优质储层连续发育的特征,台内区丘滩相沉积不足、岩溶作用效果变差,储层厚度减薄,品质降低,常规手段在对此类储层性质的研究上具有一定的局限性.为此,应用X射线CT扫描成像与数字岩心技术,以川中磨溪—龙女寺台内地区震旦系灯影组四段白云岩岩溶储层为例,将储集空间分为孔隙和喉道,从定性和定量的角度进行储层系统表征.结果表明:研究区储层溶蚀孔隙、溶蚀孔洞、裂缝相间发育,裂缝切割相邻孔洞对提高储层连通性具有重要作用.孔喉相互位置关系以及孔喉半径、长度、体积等参数共同决定了储层品质,孔隙发育数量及孔径分布特征决定储集空间大小,喉道发育数量、喉道半径、单喉道长度分布情况影响储层中孔隙沟通能力即决定了储层有效性.磨溪东—龙女寺台内区裂缝发育区孔洞型储集空间是台内优质储层关键勘探目标.

为探讨华北地区中元古界洪水庄组黑色泥页岩物源和沉积环境,采集了燕辽地区清河剖面洪水庄组样品,进行元素地球化学测试和分析.结果表明:洪水庄组沉积物来源不仅有陆源碎屑物质,还有海水沉积物贡献.Y/Ho和ΣREE交会图版分析认为海水沉积物来源占比为10％～20％.此外,稀土元素分布模式以及Ce和Eu异常表明沉积物来源有火山热液活动的参与.Mo-EF/U-EF分析结果表明洪水庄为海洋非滞留海盆沉积环境,氧化还原条件在沉积期发生了演变:在洪一段沉积时期,水体还原程度强,为厌氧-硫化环境;洪二段沉积时期,水体还原程度降低,为贫氧-厌氧环境.分析认为洪水庄组物源组成和沉积环境演变与沉积构造背景密切相关.

近年来,在四川盆地川中古隆起和外围斜坡区埃迪卡拉晚期地层(震旦系灯影组)天然气勘探相继取得了重大发现和突破.深化埃迪卡拉晚期地层和沉积等方面研究,对四川盆地非继承性构造斜坡区灯影组勘探具有重要指导意义.综合利用地质、地球物理测井和地球化学资料,并结合前人研究成果,对川中古隆起和川北斜坡区灯影组沉积晚期地层SQ4进行IV级层序划分和对比,探讨了体系域的沉积演化.结果表明:1)SQ4可以划分为5个IV级层序.受沉积古地貌影响,川北斜坡区发育SQ4-1、SQ4-2和SQ4-3,古隆起高部位区发育SQ4-3、SQ4-4和SQ4-5;2)川北斜坡区灯影组SQ4由下向上为低位体系域和海侵体系域沉积,低位体系域为多期丘核—凝块石滩沉积演化,海侵体系域为丘间—丘核—砂屑滩—丘盖沉积演化.川中古隆起高石梯—磨溪地区SQ4由下向上为海侵体系域和高位体系域沉积,其中海侵体系域与川北斜坡区为等时沉积,高位体系域为丘间—砂屑滩—凝块石滩—丘坪沉积演化;3)川北斜坡区灯四段低位域具备形成岩性圈闭条件,其中SQ4-1、SQ4-2两套低位域面积分布大,是岩性圈闭勘探的有利区.

德阳—安岳台内裂陷作为四川盆地重要控藏地质单元,其成因演化认识上存在较大分歧.通过野外地质研究,发现盆地西北部上震旦统灯影组发育深色泥晶灰岩、白云岩夹薄层硅质岩的深水相沉积,与邻区灯影组巨厚台地边缘微生物丘滩复合体呈巨大岩相差异,揭示了灯影组四段深水盆地相—斜坡相—台缘丘滩相—局限台地相完整相序,进一步明确了灯影期德阳—安岳裂陷的性质,提出晚震旦世盆地西北部地区已具台—盆分异沉积格局,但盆地中部高磨地区裂陷内灯影组发育藻白云岩浅水沉积,裂陷内外地层厚度差异为后期岩溶侵蚀作用结果.结合区域构造背景、地质、地震综合研究提出:①现今德阳—安岳裂陷及周缘灯影组厚度差异是"早期伸展作用下的沉积相分异、桐湾期岩溶侵蚀改造"的地质叠加效应,其性质上具有"北段沉积作用主导,中、南段岩溶侵蚀作用主导"的分段性;②盆地中西部地区灯影组构造分异控制地貌分异,发育深水相沉积及灯二段、灯四段两期巨厚的沉积型台缘带,呈北东走向,而盆地中南部地区灯影组发育侵蚀洼地侵蚀台地,呈近南北向展布;③剖析盆地中西部地区灯影组构造沉积分异作用的油气聚集效应,该区是裂陷中心生烃区、沉积型台缘带的有利叠合区,具有生、储、盖成藏三要素包裹式的耦合特征,有利于灯影组大规模的油气聚集.

四川盆地川中古隆起磨溪北斜坡区上震旦统灯影组具有良好的勘探前景,而关于北斜坡构造演化背景下的成藏条件研究极少论及,直接制约了天然气勘探的突破和发现.为此,从钻井、地震资料分析入手,恢复磨溪北斜坡灯影组构造演化过程,并探讨构造演化对烃源岩和圈闭成藏条件的控制作用,以期为川中震旦系天然气扩展勘探提供理论支撑.结果表明:①磨溪北斜坡经历了早期古隆起—晚期斜坡的构造演化过程.桐湾运动末期,磨溪北斜坡处于古隆起构造高部位,海西晚期沉积前受伸展裂陷作用下沉.印支晚期至现今,受前陆造山运动影响,形成现今构造斜坡区.②受加里东早期盆内伸展裂陷作用,川北地区形成裂陷槽,同时在磨溪以北形成斜坡相沉积,是下寒武统烃源岩在裂陷槽北部和斜坡腹部优质分布的重要控制因素.③灯影组沉积期,磨溪北斜坡处于古隆起构造高部位,北斜坡台缘区灯影组发育岩溶丘滩相沉积和岩性圈闭,后期稳定继承性斜坡构造演化有利于岩性圈闭保存.

针对鄂尔多斯盆地大牛地气田马五段低孔低渗岩溶白云岩储层发育特征,展开对致密岩溶白云岩储层孔隙成因进行研究.通过岩心、薄片观察,结合扫描电镜、CT扫描分析,明确了白云岩储层岩石学类型、孔隙类型及成因.结合测井及地球化学分析,认为表生期孔隙主要为膏溶孔、溶孔及地开石晶间孔隙;白云石含量及结构控制了白云岩晶间孔隙;残留有机质保护了藻云岩残余晶间孔隙.通过岩溶白云岩孔隙成因分析,最后认为:(1)白云石化作用及成因模式是造成岩溶白云岩储层低孔低渗的主要原因,准同生期-浅埋藏期形成的白云石晶粒普遍较为细小,晶粒结构以半自形-它形为主,晶间孔隙为微孔,造成孔隙喉道较窄;泥微晶白云岩与粉-细晶白云岩(储集岩)分布在纵向上具有不均匀性,造成白云岩整体上孔渗较低;(2)岩溶作用增加了白云岩储层的非均质性,裂缝、溶洞及较大扩溶孔被泥质、方解石充填,很大程度上降低了岩溶白云岩储层的孔渗性能,正因为膏质泥微晶白云岩和粉-细晶白云岩为低孔低渗特征,使得地表水携带的泥质不易充填孔隙.

Great breakthrough recently achieved in the Sinian–Lower Paleozoic gas exploration in the Leshan–Longnüsi paleo-uplift, Sichuan Basin, has also made a common view reached, i.e., large-scale paleo-uplifts will be the most potential gas exploration target in the deep strata of this basin. Apart from the above-mentioned one, the other huge paleo-uplifts are all considered to be the ones formed in the post-Caledonian period, the impact of which, however, has rarely ever been discussed on the Sinian–Lower Paleozoic oil and gas reservoir formation. In view of this, based on outcrops, drilling and geophysical data, we analyzed the Sinian–Lower Paleozoic tectonic setting and sedimentary background in the East Sichuan Basin, studied the distribution rules of reservoirs and source rocks under the control of paleo-uplifts, and finally discussed, on the basis of structural evolution analysis, the conditions for the formation of Sinian–Lower Paleozoic gas reservoirs in this study area. The following findings were achieved. (1) The Dazhou–Kaijiang inherited uplift in NE Sichuan Basin which was developed before the Middle Cambrian controlled a large area of Sinian and Cambrian beach-facies development. (2) Beach-facies reservoirs were developed in the upper part of the paleo-uplift, while in the peripheral depression belts thick source rocks were developed like the Upper Sinian Doushantuo Fm and Lower Cambrian Qiongzhusi Fm, so there is a good source–reservoir assemblage. (3) Since the Permian epoch, the Dazhou–Kaijiang paleo-uplift had gradually become elevated from the slope zone, where the Permian oil generation peak occurred in the slope or lower and gentle uplift belts, while the Triassic gas generation peak occurred in the higher uplift belts, both with a favorable condition for hydrocarbon accumulation. (4) The lower structural layers, including the Lower Cambrian and its underlying strata, in the East Sichuan Basin, are now equipped with a large-scale uplift with a simple structural configuration, above which a complete local structural trap was well developed with a good preserving condition. In conclusion, there are good forming conditions for hydrocarbon reservoirs in the Sinian–Lower Paleozoic strata in the Dazhou–Kaijiang paleo-uplift, East Sichuan Basin, in the higher parts of which the well-developed beach-facies reservoirs occupy an area of 8000 km2 and will be the next important gas exploration target in this basin.

在小层细分、沉积、储层研究的基础上,充分利用气水分析化验和试采资料,系统分析了须二气藏气水特征.认为蓬莱地区须二气藏气水分布主要受控于储集体和构造位置,单个储集体内气水总体具有“上气下水”特征、优质储层发育的地区含气性好:气层主要分布在距须二段顶界90 m以内的T3x24、T3x25砂层组,下部T3x21、T3x22和T3x23砂层组主要为气水同层.

利用岩心薄片、岩屑薄片、测井和地震资料对四川盆地须家河组砂岩类型开展研究,明确了川西至川中地区须一、须三、须五段内大面积发育灰屑砂岩,属于三角洲前缘水下分流河道沉积,具有成熟度低、灰质胶结重、粒间孔少、物性差等特征,局部发育相对高孔“甜点”砂体.电性上表现为“三低两高”特征,即低自然伽玛、低声波时差、低补偿中子、高电阻、高密度;地震上表现为高阻抗特征.以灰屑砂岩作为典型标志层,提出在剑阁—蓬莱地区发育大型三角洲体.该三角洲体具有继承性发育特征,在须一、须三、须五段沉积时期,与优质泥质烃源岩共生,生储盖组合好,具备形成大规模致密气的成藏条件.图6参4

Based on large amounts of data of lithological characters,physical property, pore structure and thin section, we paraphrased controlling factors of the reservoir development in combination with sedimentary elements and diagenesis. The results indicate that Denglouku Formation sandstone belongs to braided delta plain subfacies in Changling gas field, composed of feldspathic lithic sandstone, the reservoir space consists in the intergranular dis- solved pores, associated with low porosity and permeability. Rock source is the basis of sand bodies, the physical properties of quartz sandstone are better than that of lithic quartz sandstone and lithic sandstone. Both sedimentary evolution and position of secondary pores control the distributive intervals of effective reservoir. The plane hetero- geneity is constrained by sedimentary mierofacies and diagenetic facies. Strong compaction and cementation are the dominant factor to create the low permeability reservoirs,in contrast that dissolved secondary pores are in favor of relatively high permeability zones so as to make the low permeability reservoir become better.