四川盆地东部地区下志留统龙马溪组页岩储层特征

Characteristics of the shale gas reservoir rocks in the Lower Silurian Longmaxi Formation,East Sichuan basin,China

四川盆地是中国西南部重要的含油气盆地,在东部和南部地区下志留统龙马溪组页岩广泛发育。在川东南、鄂西渝东地区的勘探井中志留系具有良好的气显示。研究区龙马溪组厚65～516m,底部为一套海侵沉积的富含笔石的黑色页岩,龙马溪组向上和向东砂质和钙质含量增加,演变为浅水陆棚沉积。龙马溪组主要由层状-非层状泥/页岩、白云质粉砂岩、层状钙质泥/页岩、泥质粉砂岩、层状-非层状粉砂质泥/页岩、粉-细粒砂岩、钙质结核、富含有机质非层状页岩8种岩相组成。总有机碳含量(TOC)为0.2%～6.7%。有机质以II型干酪根为主,Ro为2.4%～3.6%。页岩中石英矿物含量在2%～93%,主要呈纹层状或分散状分布,主要为陆源碎屑外源成因。龙马溪组页岩岩心孔隙度为0.58%～0.67%,渗透率为0.01×10-3μm2～0.93×10-3μm2。扫描电镜下龙马溪组页岩微孔隙度为2%左右,主要包括晶间孔和粒内孔,孔隙直径为100nm～50μm。页岩储层的形成机理主要为有利矿物组合、成岩作用和有机质热裂解作用。龙马溪组与美国Barnett页岩具有一定差异,主要表现在龙马溪组页岩埋藏较深、热演化程度较高、含气量较低、储层较致密、以陆源成因石英为主。对于评价下志留统龙马溪组页岩气勘探前景而言,今后须重点加强针对龙马溪组底部黑色硅质岩系石英成因、成熟度、埋藏史、含气量等方面的研究,以及进行详细的古地貌和古环境恢复。

The Sichuan basin is an oil-bearing and gas-rich basin with extensive development of the Lower Silurian Longmaxi Formation shale in southwestern China.The gas shows in the shale were identified in exploration wells mainly located between southeastern Sichuan basin and western Hubei-eastern Chongqing.The thickness of the Silurian Longmaxi Formation shale ranges from 65 to 516m.The base of the Longmaxi Formation shale is graptolite-rich transgressive black shale.Its thickness increases eastward in the study area,similarly as the sand content in the formation,with the latter also increasing stratigraphically upward.The Longmaxi Formation is comprised by eight lithofacies,including laminated and nonlaminated mudstone/shale,dolomitic siltstone,laminated lime mudstone/shale,argillaceous siltstone,laminated and nonlaminated silty mudstone/shale,fine grained silty sandstone,calcareous concretions and nonlaminated shale enriched organic matter.Longmaxi Formation contains 0.2% to 6.7% of organic carbon（TOC）.The organic matter is overmature,with Ro 2.4%~3.6% and dominated by type II-kerogen.Quartz silt,which is the second important component of the shale gas reservoir quality,occurs as laminae and/or disseminated and varies from 2%~93% in the shale.The size of quartz silt ranges from 0.03 to 0.05mm,with terrigenous origin.Porosity measured on core samples of the shale from the Longmaxi Formation in exploratory wells ranges from 0.58% to 0.67%.The microporosity observed in the thin sections of the shale is about 2%,and dominated by the intercrystal and intragranular pores,with the pore size ranging from 100nm to 50μm.The formation mechanism of the shale reservoirs includes favorable mineral composition,diagenesis and thermal cracking of organic component.There are some differences between Longmaxi Formation shale and Barnett shale in USA.The former is burial deeper,higher degree of thermal evolution,lower gas content,denser,more quartz of terrigenous origin.The prevailing low content of organic matter and highly variable quartz content in the Longmaxi Formation shale suggests there are only marginal conditions for exploration of shale gas resource.However,the high variability in both the content of TOC and quartz in the shale indicates that locally,particularly in the southeastern part of the basin,favorable conditions for shale gas may have developed.More detailed paleogeographic,burial history,gas content and quartz origin studies are needed to better access shale-gas potential of the Silurian Longmaxi Formation shale.