Comparison of basic features and origins of oolitic shoal reservoirs between carbonate platform interior and platform margin locations in the Lower Triassic Feixianguan Formation of the Sichuan Basin,southwest China

The oolitic shoal reservoirs of the Lower Triassic Feixianguan Formation carbonates in the Sichuan Basin of southwest China are an important target for gas exploration in the basin.Their occurrence,like other cases worldwide,can be divided into two locations in general,i.e.,platform interior and platform margin locations.Their differences of reservoir features and origins,however,have not been investigated comprehensively due to different exploration degrees.This issue is addressed in this paper,to provide basic data and information for the basin＇s hydrocarbon exploration and for the study of carbonate platform sedimentology and reservoir geology worldwide.We compared the features of these two types of reservoirs in detail,including the depositional and diagenetic features,pore types and petrophysical features.Based on the comparison,the origin of the reservoirs was further discussed.It is shown that the reservoirs in platform interior and platform margin locations differ significantly.The interior carbonates were deposited in moderate to high energy settings and the dominant lithologic type was limestone,which was weakly compacted and intensely cemented and has undergone meteoric dissolution.Pore types include intragranular dissolution and moldic pores,with low porosities（6%） and low permeabilities（0.1 mD）.By contrast,the platform margin carbonates were deposited in relatively high energy settings and mainly consisted of dolostones with some limestones.The rocks were strongly compacted but incompletely cemented.As a result,some primary intergranular pores were preserved.Both meteoric solution and burial solution have taken place.There are various types of pore spaces including intergranular and intercrystalline solution pores and residual intergranular pores.This type of reservoir generally has better petrophysical properties（9% porosity and 0.1 mD permeability） and pore-throat structures than the interior reservoirs.These differences were influenced by both primary depositional features and secondary diagenesis.For the interior carbonate reservoirs,early meteoric dissolution,weak compaction and strong cementation are important controlling factors.By contrast,the factors controlling the formation of the margin carbonate reservoirs mainly include dolomitization,preservation of primary pores and burial dissolution.

Digitized Outcrop Geomodeling of Ramp Shoals and its Reservoirs： as an Example of Lower Triassic Feixianguan Formation of Eastern Sichuan Basin

Numerous hydrocarbon accumulations are found in ramp crest shoals worldwide and therefore this depositional setting has a high potential of being the hydrocarbon reservoir. In this paper, we combined digital outcrop geology and traditional geological mapping to build an outcrop-based geocellular model of the ramp-crest shoal complex of the Lower Triassic Feixianguan Formation in the Eastern Sichuan Basin. The outcrop model serves as an analogue for the subsurface reservoir of the Feixianguan Formation and illustrates the complexity of the lithofacies types, stratigraphic architecture, and reservoir heterogeneities at a scale below conventional subsurface data resolution. The studied ramp -crest shoal complex consists of thirteen types of lithofacies that can be grouped into three facies-groups corresponding to subtidal intraclastic shoal, sub- to inter-tidal oolitic shoal, and tidal flat depositional environments respectively. The stratigraphic architecture of the shoal complex shows mostly a strong progradation of the high energy facies associated with an overall decrease of accommodation space associated with relative sea level still stand. Two reservoir facies associations have been recognized. The first one consists of supratidai dolomudstone and upper intertidal partially dolomitized oolitic packstone with anhydrite or nodules. These facies were deposited above the high energy oolitic grainstones and occurs as thin-bedded and laterally continuous layers, characterized by high porosity and low permeability. The second reservoir facies association is composed of intertidal crystalline dolomite and subtidal intraclastic bindstone that occurs stratigraphically below the oolitic grainstones. These deposits consist of massive laterally discontinuously beds, and are characterized by high porosity and high permeability. Both types of reservoir facies tend to be stacked vertically and migrated laterally with the progradation of the shoal complex. The construction of the outcrop-based 3D geological model provide a description and quantification of the facies distribution within a robust stratigraphic framework and the style and amount of reservoir heterogeneities associated with a ramp-crest shoal complex reservoir such as the one found in Lower Triassic Feixianguan Formation and Cambrian Longwangmiao Formation in Sichuan Basin or other ramp-crest reservoir worldwide.

The Major Controlling Factors and Different Oolitic Shoal Reservoir Characteristics of the Triassic Feixianguan Formation,Eastern Longgang Area,NE Sichuan Basin,SW China

Based on comprehensive analyses of occurrence,petrological observation,pore structure and geochemistry,the different reservoir characteristics and reservoir evolutionary pathways between different oolitic shoal reservoir types of the Feixianguan Formation on the west side of the Kaijiang-Liangping Trough have been studied.There exist three stages of high-energy slope break belts in the Feixianguan period,the corresponding three stages of oolitic shoals gradually migrating in the direction of the trough.Three types of oolitic shoal reservoirs,namely,residual-oolitic dolomite,mold-oolitic dolomite and sparry oolitic limestone,were formed during sedimentary-diagenetic evolution,the pore types being intergranular dissolved pore,mold pore(or intragranular dissolved pore)and residual intergranular pore,respectively.The petrology,physical properties and pore structure of the different types of oolitic shoal reservoirs are quite different.Residual-oolitic dolomite reservoirs have the best quality,while sparry oolitic limestone reservoirs have the poorest.Combined with analyses of trace elements,rare earth elements and carbon-oxygen isotopes,it is suggested that the formation of residual-oolitic dolomite reservoirs is jointly controlled by penesaline seawater seepage-reflux dolomitization and hydrothermal dolomitization.Mold-pore oolitic dolomite reservoirs are controlled by penesaline seawater seepage-reflux dolomitization and meteoric water solution.The burial dissolution of organic acid not only further improves the reservoir qualities of previously formed oolitic dolomite reservoirs,but also preserves residual intergranular pores in the sparry oolitic limestone reservoirs.

Sedimentary diagenesis of rudist shoal and its control on reservoirs: A case study of Cretaceous Mishrif Formation, H Oilfield, Iraq

Based on the core, cast thin section, whole rock analysis, conventional physical properties and high pressure mercury intrusion test, the sedimentary diagenesis characteristics of rudist shoal in Cretaceous Mishrif Formation of H Oilfield, Iraq and its control on the reservoir were studied. The rudist shoal of the Mishrif Formation develops in the high-stand systems tract and is distributed in the high places of paleogeomorphology on the edge of platform with strong hydrodynamic force. According to the relative sea level changes, lithologic evolution and sedimentary structure characteristics of the rudist shoal, the single rudist shoal is divided into four lithologic sections: A, B, C and D, that is, low-angle cross-bedding pelletoids-rudist packstone, low-angle cross-bedding and parallel bedding arene-rudist grainstone, parallel bedding rudist gravel limestone, and horizontal bedding carbonaceous mudstone. The complete sedimentary sequence of a single rudist shoal is often disrupted. Several rudist shoals superimpose to form thick rudist shoal sediment. The single rudist shoal thickness and lithologic sections assemblage change regularly in vertical direction. The rudist shoal has the characteristics of "strong dissolution, weak cementation and strong compaction", forming pore-type reservoir with intergranular pores, intergranular dissolved pores, mold pores, and dissolved pores. With mainly coarse pore throats larger than 5 μm, the reservoir is of medium-high porosity and high permeability. There is lithological reverse cycles inside single shoals and between single shoals, with content of mud crystals decreasing from the bottom to the top, dissolution increasing, cementation decreasing in strength, pore throats getting larger, and physical properties turning better. The rudist shoal of MB2-1 at the top of the high-stand systems tract has the largest thickness, moreover, subject to the strongest atmospheric freshwater leaching, this layer has the most significant dissolution and the largest pore throat, so it is the best reservoir of the Mishrif Formation.

Characteristics and main control factors of Ordovician shoal dolomite gas reservoir in Gucheng area,Tarim Basin,NW China

Based on seismic, drilling data and experimental analysis, the characteristics and main controlling factors of shoal dolomite gas reservoir in the third member of Ordovician Yingshan Formation of Gucheng area, Tarim basin were examined.The study shows that the dolomite gas reservoir in Gucheng area is lithologic gas reservoir controlled by shoal and fault jointly,and its formation is mainly attributed to the following factors:(1) The continuously developing paleotectonic structure has been in the direction of gas migration and accumulation;(2) The large area of medium-high energy grain bank deposited in gentle slope environment is the material basis for the formation of dolomite reservoir;(3) Atmospheric water leaching and dolomitization and fluid dissolution in fault zone are the key factors for the formation of high-quality dolomite reservoir;(4)The natural gas comes from cracking of the ancient oil reservoir and hydrocarbon generation of dispersed organic matter in source rocks, and the NNE-trending strike-slip fault is the dominant channel for vertical migration of natural gas;(5) Limestone cap rocks in the first and second members of Yingshan Formation provide direct sealing for the formation of gas reservoir there. On the basis of comprehensive analysis, it is pointed out that the Gucheng area has three grain shoal zones in the third member of Yingshan Formation in nearly S-N direction, which together with seven strike-slip fault zones in NNE direction control the development of shoal dolomite gas reservoir.

Reservoir Prediction Restricted by Sequence Stratigraphy--A Case Study of Bioclastic Shoal Reservoir in Changxing Formation of Jiannan Area

Bioclastic shoal reservoir in Changxing Formation of Jiannan area is characterized by small thickness and strong heterogeneity. The uncertainty of the reservoir distribution pattern has confined the effective development of this area, so the accurate bioclastic shoal reservoir prediction would be the key to achieve development breakthroughs. Based on drilling, well-log, seismic and core analysis data, this article conducted exquisite sequence stratigraphic classification and established isochronal regional stratigraphic framework of Changxing Formation in Jiannan area. The reservoir seismic corresponding features were determined by exquisite calibrating bioclastic shoal reservoir in Changxing Formation. Therefore, seismic processing methods, such as multiple attribute analysis and amplitude inversion, were applied to attain more reliable reservoir prediction results, which indicated the distribution of vertical reservoir in SSQ2, the IV sequence order and the distribution of horizontal reservoir around Well J43 and JZ1 in the platform margin of the study area.

四川盆地东部地区二叠系茅口组孤立浅滩的发现及天然气勘探潜力

中二叠统茅口组长期以来一直是四川盆地天然气勘探的重点层系之一,茅口组天然气勘探开发先后以四川盆地南部地区岩溶缝洞型石灰岩储层和四川盆地中部地区滩相白云化储层为主要目的层系。为了寻找并落实茅口组下一个天然气勘探方向,在勘探程度相对较低的四川盆地东部(以下简称川东)地区开展了大量基础研究工作,在重新梳理地层格架的基础上,对茅口组沉积、储层及成藏条件进行了系统分析,指出了该区孤峰段硅质岩下茅二下亚段孤立浅滩的存在,最后明确了茅口组天然气的勘探潜力。研究结果表明:(1)川东地区在茅口组孤峰段事件性硅质岩沉积之前,宣汉—万州地区茅二下亚段发育碳酸盐岩孤立浅滩,面积可达6500 km^(2);(2)在宣汉—万州地区茅二下亚段发现了滩相基质孔储层,其中发育于茅二下亚段底部的孔隙型白云岩储层厚3~12 m,发育于茅二下亚段中上部的孔隙型石灰岩储层厚5~18 m;(3)川东地区茅口组滩相储层成藏条件优越,其中烃源岩主要为下伏上奥陶统五峰组—下志留统龙马溪组厚层页岩,且与烃源岩沟通的断裂发育,区域盖层主要为孤峰段硅质岩及吴家坪组海相泥岩、页岩,保存条件良好。结论认为,川东地区茅口组孤立浅滩的发现和良好的成藏组合条件,将进一步加快该区天然气勘探开发进程,茅口组大面积分布的高能浅滩有望成为下一个天然气勘探开发重要领域。

冲刷及整治工程叠加作用下武汉河段滩槽调整特征与成因

大型水库的修建引起了坝下游的滩槽调整,进而对河势、航运、防洪及生态等产生显著影响。为探究滩槽调整的驱动机制,采用1952—2022年水沙和1959—2021年地形观测资料,以长江武汉河段为例,分析了床沙组成、来水来沙及整治工程等因素的影响。研究表明:①武汉河段枯水河槽冲刷量占全河槽冲刷量的90.4%,断面趋向窄深化,河相系数减小。不同江心洲及汊道冲淤出现差异性调整,2001—2021年,铁板洲和白沙洲洲体面积分别减少了63.1%和61.5%,潜洲和天兴洲洲体面积分别增大了212.3%和14.5%;2013年后,铁板洲左右汊均冲,天兴洲汊道左淤右冲。②来沙量锐减,河床以0.125 mm≤d<0.25 mm的细沙居多,抗冲性较弱,是冲刷加剧的先决条件;三峡水库的削峰补枯作用加剧了枯水倾向汊河的冲刷幅度;整治工程改变了滩槽冲淤的横向分布,是江心洲差异性调整的主导因素,也是天兴洲保持左淤右冲的重要原因。③三峡水库蓄水前,洲滩之间演变联动性强;三峡水库蓄水后,来沙减少和中水期时间延长导致洲滩冲刷萎缩,洲滩之间演变联动性变弱;整治工程实施后,工程保持了心滩稳定且其缓流滞沙效应提高了抗冲性,心滩淤长的同时限制了边滩发育,叠加来沙进一步减少,边滩之间、心滩之间演变联动性进一步变弱,边滩与心滩之间演变联动性再次增强。

梁平地区二叠系长兴组台缘礁滩储层演化——来自天东110井岩石学及地球化学证据

天东110井是川东地区长兴组近年来获得的高产稳产井之一,该井的不同类型储层成因研究及模式建立对梁平地区长兴组勘探具有重要指示作用。基于天东110井长兴组丰富的取心资料,文中对其开展了岩石学、地球化学分析,建立了不同储集空间的演化模式,提出了高产井储层分布模式。结果表明:1)天东110井长兴组发育孤立状洞、溶洞、孔隙(包括针状孔及晶间孔)、高角度构造溶蚀缝。2)天东110井长兴组针状孔分布于礁滩体的颗粒云岩中,准同生期大气淡水溶蚀成孔、渗透回流白云岩化起关键保孔作用。3)缝洞形成受晚三叠世诺利期地层水溶蚀及瑞提期热液溶蚀作用控制,前者被沥青及巨晶方解石全充填,储集性能较差,后者部分被方解石、白云石、萤石等充填,充填程度低,孔渗性能好。下一步勘探应注重台地边缘的白云岩礁滩体微地貌高点与近晚三叠世瑞提期断裂及裂缝的热液溶蚀叠合发育区,可望获得更大突破。

鄂尔多斯盆地东北部奥陶系马四段白云岩储层特征及主控因素

米探1井等多口探井在鄂尔多斯盆地东北部奥陶系马四段白云岩中获得高产天然气流,展示出该区域马四段具有良好的勘探前景。然而对盆地东北部马四段是否发育规模白云岩储层还存在争议,白云岩储层分布不清制约了下一步勘探部署。基于岩心、薄片、场发射扫描电镜及地球化学特征等资料,对盆地东北部奥陶系马四段白云岩储层特征及其发育控制因素开展了系统研究,并预测了盆地东北部奥陶系马四段白云岩储层分布。研究结果表明:(1)盆地东北部奥陶系马四段白云岩发育潮坪型白云岩储层、丘滩型白云岩储层和生物扰动型白云岩储层,3类白云岩储层储集空间主要为晶间孔,含少量微裂缝,平均孔隙度分别为4.77%、5.12%和2.52%。(2)白云岩储层的形成与丘滩体、生物扰动作用和白云石化作用相关。白云石化作用是潮坪型白云岩储层晶间孔形成的关键,丘滩体和白云石化作用控制了丘滩型白云岩储层发育,生物扰动作用和白云石化作用控制了生物扰动型白云岩储层发育。(3)盆地东北部神木—靖边地区在奥陶纪马四期位于低隆起带上,有利于丘滩体发育和生物扰动作用、白云石化作用的发生,形成的白云岩储层可为天然气富集提供规模储集空间。

川东地区寒武系洗象池组构造特征及成藏模式

四川盆地东部上寒武统洗象池组是盆地内下组合油气勘探的重点领域。基于地层及沉积演化特征,利用地震、钻井岩心以及地球化学等资料,分析了川东地区上寒武统洗象池组构造特征及变形机制,从源、储、盖、运4个方面分析了其油气成藏条件,并总结了成藏模式。研究结果表明:(1)川东地区寒武系洗象池组表现为以2套滑脱变形带(中寒武统高台组膏盐岩和志留系泥岩)控制的、沿北西向传递的断褶构造,发育背冲构造、叠瓦状构造、断弯褶皱和盐底辟断弯褶皱4种构造圈闭样式;中寒武统高台组膏盐岩和中—下奥陶统泥灰岩2套盖层分别将下寒武统筇竹寺组、上奥陶统五峰组—下志留统龙马溪组2套烃源岩与洗象池组隔离。(2)研究区洗象池组具备油气成藏条件,五峰组—龙马溪组泥页岩是主要烃源岩,厚度为300~700 m,TOC值为2%~7%,生烃能力强,Ro为2.4%~4.0%,处于过成熟阶段;洗象池组沉积后期被岩溶和裂缝改造的滩相白云岩是相对优质的规模较大的储层,孔隙度大于2.1%;燕山期构造抬升和变形不仅在洗象池组形成了有效构造圈闭,还使其突破中—下奥陶统盖层与五峰组—龙马溪组对接,实现了油气的水平侧向运移。(3)研究区洗象池组为“新生古储、源储并置、侧向运移、构造圈闭”晚期天然气成藏模式;逆冲断层导致上盘褶皱的洗象池组与下盘五峰组—龙马溪组在水平面上并置,形成了源-储横向对接的构造圈闭,有效地提高了供烃和成藏的效率。

川中-蜀南地区茅口组岩溶储层油气成藏条件与有利区带预测

四川盆地茅口组早期勘探一直聚焦蜀南和川东高陡构造带及裂缝发育区,近年来老井复查揭示川中-蜀南地区非构造高部位,特别是斜坡-向斜区的油气显示活跃,但是斜坡-向斜区油气储层分布及发育模式认识不清,区域茅口组岩溶储层油气勘探部署方向不明确。利用岩心观察、CT成像、测井解释、地震预测等方法,针对川中-蜀南地区茅口组岩溶储层开展攻关。结果表明:茅口组发育2套颗粒滩体,下部茅二段b亚段颗粒滩发育一种受水平潜流带控制的岩溶储层新类型,即孔隙-孔洞型,与茅口组上部茅二段a亚段至茅三段颗粒滩发育的裂缝-孔洞型岩溶储层一起,构成了川中-蜀南地区茅口组的有效岩溶储层;茅口组岩溶储层经历了3期油气充注过程,具有“现今背斜构造区早期规模富集、晚期差异调整,斜坡-向斜区原位保存”大面积成藏的特点;结合关键成藏期古地貌、颗粒滩体和现今构造,最终优选威远—古蔺一带附近茅口组2个有利勘探区带,同时建议油气勘探目标可从传统的仅为上部的裂缝-孔洞型岩溶储层类型拓展到上下2套储层兼顾,特别是不发育断裂且构造稳定的向斜区,新发现的孔隙-孔洞型碳酸盐岩储层是茅口组油气勘探的新领域,本研究可为四川盆地茅口组油气勘探提供新的思路。

川中古隆起北斜坡寒武系龙王庙组油气成藏特征及演化模式

利用测井、录井、岩心分析化验等资料,对川中古隆起北斜坡下寒武统龙王庙组油气成藏条件、油气充注特征及成藏演化模式展开了深入研究。研究结果表明:(1)川中古隆起北斜坡下寒武统龙王庙组天然气具备良好的成藏条件,下伏筇竹寺组烃源岩在北斜坡腹部及西侧裂陷槽存在2个生烃中心,其TOC平均值分别为1.90%和3.46%,有机质以Ⅰ型干酪根为主,可提供充足且优质的烃源;龙王庙组内部发育的颗粒滩体岩溶储层与上覆高台组厚层致密泥质云岩,构成了优势储-盖组合;区内广泛发育的油源断裂可作为纵向优势运移通道高效输导油气。(2)研究区龙王庙组具3期成藏特征:晚三叠世末期(约214~206 Ma)发生第1期充注,大量成熟油聚集形成古油藏;早侏罗世末期(约193~186 Ma)发生第2期充注,表现为“多油少气”混合充注;中侏罗世末期(约170~164 Ma)发生第3期充注,大量油裂解气充注储层并聚集形成现今气藏。(3)研究区龙王庙组油气成藏演化可划分为4个阶段:加里东早期—二叠纪前少量低熟油充注阶段、二叠纪—三叠纪大量成熟油充注形成古油藏阶段、中侏罗世—白垩纪古油藏被破坏形成裂解气藏阶段以及晚白垩世至今气藏调整阶段。

四川盆地PS8井区灯影组四段丘滩体储层特征及成因机制

四川盆地震旦系灯影组四段是近期的重点勘探层位,PS8井灯影组四段试气获得高产,展示出灯影组四段天然气勘探的巨大潜力。基于钻井岩心、岩屑、薄片、分析化验和测井等资料,对PS8井区灯四段丘滩体储层特征及成因机制开展研究。结果表明:①PS8井区灯影组四段从裂陷槽向台内具有逐渐增厚的趋势,储集层岩性主要为藻凝块白云岩、藻叠层白云岩、砂屑白云岩;储集空间以溶蚀孔洞及裂缝为主,柱塞样孔隙度为0.86%~6.59%,平均3.58%,渗透率为0.00085~14.75mD,平均1.71mD,全直径孔隙度为1.15%~8.42%,平均3.84%,渗透率为0.003~16.8mD,平均2.16mD,属于低孔低渗型储层;②台缘带发育的微生物丘滩沉积是PS8井区灯影组四段优质储层发育的物质基础,表生期风化壳岩溶和埋藏期热液溶蚀作用是PS8井区灯影组四段储层大量溶蚀孔洞发育的关键因素。研究成果落实了研究区灯影组四段丘滩体储层特征,并明确了储层的成因机制,为后期储层发育分布预测提供了理论支撑,同时为评价方案的部署提供了地质依据。

四川盆地蓬溪-武胜台凹西侧长兴组成藏主控因素

蓬溪-武胜台凹为开江-梁平海槽同期构造产物,其两侧的油气勘探一直未取得较大突破,成藏特征认识也不够清晰。文中在系统梳理烃源岩、储层、构造等成藏条件基础上,结合最新钻探成果,开展了成藏主控因素研究。结果表明,长兴组成藏具有3个基本条件,即多套优质烃源岩的发育,规模发育的礁滩储层,通源断层提供的油气垂向进入长兴组的通道。长兴组油气富集规律取决于2个要素:1)源储匹配。源储匹配不佳决定了长兴组未能大规模成藏,多套烃源岩中仅筇竹寺组在生、排烃期上与断层活动期的匹配较好,其余烃源岩对长兴组供烃有限;由于延伸至长兴组的断层较少,源储间有效通道缺乏又进一步导致了气源不足。2)生物礁滩储层发育状况。生物礁滩储层的连续发育促成了垂侧向复合输导体系的形成,在此基础上有限的烃类会逐步向斜坡高部位运移聚集,气水分布受构造影响较大,非“一礁一藏”。据此研究结果,优选了4个勘探有利区。

印模法古地貌恢复在岩溶丘滩相有利区评价中的应用:以川中古隆起北斜坡灯影组二段为例

四川盆地寒武系-震旦系裂陷槽东侧川中古隆起北斜坡灯影组二段岩溶丘滩相储层非均质性强,主控因素复杂,成为制约北斜坡地区下一步天然气勘探目标优选的关键因素。利用三维连片地震数据,基于最新探井钻井、测井等资料,创新思路,提出用灯影组沉积后期的岩溶地貌来刻画滩相储层有利区,把上覆地层沧浪铺底作为基准层,采用印模法进行微幅古地貌分析,刻画了灯二岩溶期地貌特征。在此基础上,精细研究了储层的地震响应特征,建立了地震相分类模式,利用波形分类方法进行岩溶丘滩相储层发育有利区预测。成果揭示灯二段岩溶丘滩相储层发育规律呈与侵蚀槽展布方向相同的北西-南东向展布,与实际钻井和地质认识一致。相较之常规用的残厚法,该方法更加考虑了沉积特征和地质规律,减少了地震预测的多解性,且可达到半定量预测效果,为古隆起北斜坡单斜构造背景下研究岩溶滩相储层有利区提供了有利指导。

Sedimentary Facies and Distribution of Reservoir Rocks from the Feixianguan Formation in the Daxian-Xuanhan Region, NE Sichuan

The Lower Triassic Feixianguan Formation in the Daxian-Xuanhan region, northeastern Sicbuan is interpreted to be a carbonate platform intermediate between the West Hubei-East Cbongqing marine basin and Guangyuan-Wangcang marine basin. Outcrops, well logs and seismic data have disclosed that three sedimentary facies can be identified for the formation： open platform, platform exposed shoal and restricted platform facies. During the early stage of deposition of the Feixianguan Formation, there existed a sedimentary framework all the same as the Late Permian one： open platform in the west, and platform exposed shoal, restricted platform and open platform eastwards. The gradual increase of the shoal area permitted eastward migration of the shoal facies. During the deposition of the third member of the Feixianguan Formation, the study area was invaded by temporary transgressions. Till the deposition of the fourth member of the formation, the study area was prevailed over the unified restricted platform deposits, as indicated by the sedimentary model for the typical carbonate platform shoal deposits. The reservoir rocks in the Feixianguan Formation consist mostly of solution opening dolostone, and the rock types are assembled by oolitic dolostone, residual oolitic dolostone and sucrosic residual oolitic medium- to coarse-grained dolostone. The deep and shallow dual laterologs show relatively high resistivity, clear amplitude differences and time difference saltation of acoustic waves in some intervals. The seismic responses are indicated by low-frequency and highly variable amplitudes and chaotic reflection configurations. The reservoir rocks are characterized by high porosity-high permeability and medium porosity-medium permeability, showing a marked correlation between porosity and permeability. Solution openings are extremely developed in the reservoir rocks, including cast pores, intergranular solution openings, intercrystal pores, intercrystal solution openings, solution openings （caves） and minute fissures. Macropores and coarse pore throats are common. The reservoir rocks are apparently controlled by sedimentary facies in distribution patterns. Vertically, they occur dominantly in the second and first members of the Feixianguan Formation, and laterally in the platform exposed shoals and restricted platforms, where good natural gas potential is expected.

Formation mechanisms,potentials and exploration practices of large lithologic gas reservoirs in and around an intracratonic rift:Taking the Sinian-Cambrian of Sichuan Basin as an example

By examining structures,sediments,reservoirs and accumulation assemblages in the Deyang-Anyue rift and its surrounding area,four new understandings are obtained.First,during the initiation period of Deyang-Anyue rift,multiple groups of faults developed in the rift due to the effect of tensile force,bringing about multiple mound and shoal belts controlled by horsts in the second member of the Sinian Dengying Formation;in the development stage of the rift,the boundary faults of the rift controlled the development of mound and shoal belts at the platform margin in the fourth member of Dengying Formation;during the shrinkage period of the rift,platform margin grain shoals of the Cambrian Canglangpu Formation developed in the rift margin.Second,four sets of large-scale mound and shoal reservoirs in the second member of Dengying Formation,the fourth member of Dengying Formation,Canglangpu Formation and Longwangmiao Formation overlap with several sets of source rocks such as Qiongzhusi Formation source rocks and Dengying Formation argillaceous limestone or dolomite developed inside and outside the rift,forming good source-reservoir-cap rock combinations;the sealing of tight rock layers in the lateral and updip direction results in the formation model of large lithologic gas reservoirs of oil pool before gas,continuous charging and independent preservation of each gas reservoir.Third,six favorable exploration zones of large-scale lithologic gas reservoirs have been sorted out through comprehensive evaluation,namely,mound and shoal complex controlled by horsts in the northern part of the rift in the second member of Dengying Formation,isolated karst mound and shoal complex of the fourth member of Dengying Formation in the south of the rift,the superimposed area of multi-stage platform margin mounds and shoals of the second and fourth members of Dengying Formation and Canglangpu Formation in the north slope area,the platform margin mounds and shoals of the second and fourth members of Dengying Formation in the west side of the rift,the platform margin mound and shoal bodies of the fourth member of Dengying Formation in the south slope area,etc.Fourth,Well Pengtan-1 drilled on the mound and shoal complex controlled by horsts of the second member of Dengying Formation in the rift and Well Jiaotan-1 drilled on the platform margin mound and shoal complex of the North Slope have obtained high-yield gas flows in multiple target layers,marking the discovery of a new gas province with reserves of(2-3)×10^(12) m^(3).This has proved the huge exploration potential of large lithologic gas reservoir group related to intracratonic rift.

Reservoir characteristics and genetic differences between the second and fourth members of Sinian Dengying Formation in northern Sichuan Basin and its surrounding areas

This study investigated the characteristics and genesis of reservoirs in the 2^(nd) and 4^(th) members of Sinian Dengying Formation in northern Sichuan and its surrounding areas, on the basis of outcrop, drilling cores and thin section observation and geochemical analysis. The reservoirs of 2^(nd) member are distributed in the middle part of the stratum. The reservoir quality is controlled by supergene karst and the distribution of mound-shoal complex. The bedded elongated isolated algal framework solution-cave and the residual "grape-lace" cave, which are partially filled with multi-stage dolosparite, constituted the main reservoir space of the 2^(nd) member. There is no asphalt distribution in the pores. The pore connectivity is poor, and the porosity and permeability of the reservoir is relatively low. The reservoirs of 4^(th) member were distributed in the upper and top part of the stratum. The reservoir quality is controlled by burial dissolution and the distribution of mound-shoal complex. The bedded algal framework solution-pores or caves, intercrystalline pores and intercrystalline dissolved pores constituted the main reservoir space of the 4^(th) member. It's partially filled with asphalt and quartz, without any dolosparite fillings in the pores and caves. The pore connectivity is good. Most of the 4^(th) member reservoirs had medium-low porosity and low permeability, and, locally, medium-high porosity and medium permeability. Affected by the development of mound-shoal complex and heterogeneous dissolution, the platform margin along Ningqiang, Guangyuan, Jiange and Langzhong is the most favorable region for reservoir development. Deep buried Dengying Formation in the guangyuan and langzhong areas should be the most important hydrocarbon target for the future exploration.