川东地区茅一段储层地质特征及勘探潜力分析——以XT1井为例

Geological characteristics and exploration prospects of the first member of Maokou Formation in eastern Sichuan Basin:An example from XT1 well

在以往的勘探实践中,四川盆地中二叠统茅口组一段(简称茅一段)长期被视为一套碳酸盐岩烃源层,未曾针对性地开展储层评价与测试工作。为了进一步明确川东地区茅一段的天然气勘探潜力,以风险探井XT1井岩心资料为研究对象,通过物性测试、薄片鉴定、电镜扫描、有机地球化学分析等技术手段,综合开展矿物岩石学和地球化学分析,以获取川东地区茅一段的储层特征及烃源特征。研究成果表明:①川东地区茅一段以中层状、中—薄层状深灰—灰黑色含泥质泥晶生屑灰岩、泥灰岩与中厚—厚层状灰—深灰色泥晶生屑灰岩、含泥质泥晶生屑灰岩为主,沉积水体深度较大;泥晶生屑灰岩及含泥质泥晶生屑灰岩、泥灰岩互层形成的“眼球状构造”发育。②XT1井茅一段储层岩心柱塞样氦气孔隙度分布在0.24%~6.98%,平均为1.39%;渗透率分布在0.0014~1.7200 mD,平均为0.0830 mD,属于低孔隙度、低渗透率储层;其中眼皮状构造灰岩(简称为“眼皮灰岩”)物性相对较好,孔隙度分布在0.24%~6.98%,主要储集空间类型为片状滑石晶间微孔、微裂缝;而眼球状构造灰岩(简称为“眼球灰岩”)储集性能较差,孔隙度分布在0.24%~3.41%,储层不发育。③XT1井茅一段眼皮灰岩具有一定的生烃能力,总有机质含量(TOC)分布在0.17%~1.63%;川东地区茅一段烃源岩厚度多大于80 m,具有良好的生烃条件。④川东地区茅一段可形成自生自储、多元供烃的含泥质灰岩、泥灰岩气藏,勘探有利分布面积大于2×10^(4) km^(2),预测资源量超过2000×10^(8) m^(3),具有较大的勘探潜力。

The first member of Middle Permian Maokou Formation(Maokou 1 Member)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 Maokou 1 Member 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 iden-tification,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 inter-crystalline 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 Maokou 1 Member in eastern basin are generally thicker than 80 m,thus suggesting good hydrocarbon-generating conditions;(iv)in this area,this member can develop into argillaceous limestone and marlstone gas 42 reservoirs with self-generation and self-storage as well as multiple source rocks with the favorable exploration area of 2×10 km and the pre-8 dicted resources over 2,000×10 m3,displaying large exploration prospects.