Title
Development characteristics and formation mechanism of lacustrine dolomite reservoirs in the Paleogene inter-salt shale sequence, Qianjiang sag, Jianghan Basin
摘要
以江汉盆地潜江凹陷盐间页岩层系潜江组三段下亚段4油组第10韵律层（潜三韵律层）为研究对象，以钻井、岩心及测井数据为基础，结合薄片鉴定、扫描电镜、压汞、X衍射、岩石冷冻热解及饱和烃色谱-质谱等分析测试数据，对韵律层白云岩储层发育特征及形成机理进行研究。结果表明：①目的层矿物组成复杂，主要发育有富炭泥质白云岩相、富炭泥质灰岩相、富炭白云质泥岩相和富炭、钙芒硝充填云质泥岩相4种岩相类型，其中富炭泥质白云岩相为最有利岩相类型，具有有机质丰度高（有机碳含量3.3%~6.3%）、脆性矿物含量高（50%~76%）、粘土矿物含量低（18%~33%）和孔隙度高（10.8%~26.3%）的特点；②白云岩类储层储集空间以晶间孔隙为主，溶蚀孔、缝极少发育，孔隙之间的连通性差，孔隙结构不好；③泥质白云岩储层宏观分布受沉积环境控制明显，富炭泥质白云岩相主要出现在湖平面下降中期水体盐度适中的区域内，微观上，孔隙发育受压实作用控制明显，胶结和溶蚀作用影响较小；④早成岩作用阶段较弱的压实作用、准同生期白云石化作用及储层中油气的充注作用对晶间孔隙的保存具有积极的作用，是研究区储层形成的主要原因。
Abstract
This study deals with the 10 cyclotherm of the 4 oil layer group in the lower submember 3 of the member of Qianjiang Formation (hereafter referred to as Qian 3 cyclotherm), an inter-salt shale sequence in the Qianjiang sag, Jianghan Basin, with respect to the development characteristics and formation mechanism of dolomite reservoirs in the cyclotherm. The drilling, core and logging data serve as the research basis, in combination with analytical data from thin section identification, scanning electron microscopy (SEM), high pressure mercury intrusion porosimetry (MIP), X-ray diffraction, rock freezing and Rock-Eval pyrolysis, and gas chromatography-mass spectrometry (GC-MS) of saturated hydrocarbon. The results show that (1) the target layer features complicated mineral compositions of 4 lithic facies, including carbon-rich argillaceous dolomite, carbon-rich argillaceous limestone, carbon-rich dolomitic mudstone and carbon-rich dolomitic mudstone filled with glauberite. Among others, the carbon-rich argillaceous dolomite is considered the best, owing to its high abundance of organic matters in a range of 3.3% to 6.3%, high level of brittle minerals of 50% to 76%, low level of clay minerals of 18% to 33%, and high porosity of 10.8% to 26.3%; (2) The reservoir space of the dolomite reservoirs is dominated by intercrystalline pores, together with few dissolved pores and fractures, and these pores and fractures are characterized by poor structure and poor connectivity; (3) The macroscopic distribution of argillaceous dolomite reservoirs are significantly determined by sedimentary setting. For instance, the carbon-rich argillaceous dolomite mainly occurs in areas of moderate salinity during the middle phase of eustatic lake level descending. Microscopically, its pore development is markedly associated to compaction, while slightly to cementation and dissolution; (4) The weak compaction during the early diagenesis, the penecontemporaneous dolomitization and the hydrocarbon charging to reservoirs, all play a positive role in the preservation of intercrystalline pores, and are key to reservoir formation in the study area.
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