川中侏罗系大安寨二亚段页岩油富集有利岩性组合

Favorable lithologic combinations for shale oil enrichment in the second submember of Da’anzhai Member,Jurassic,central Sichuan Basin

我国湖相页岩层系地层中岩性组合复杂,且不同岩性组合中的页岩油含量差异较大,明确有利的岩性组合将有助于探寻页岩油富集区。以川中侏罗系大安寨二亚段页岩层系为研究对象,通过矿物组分分析和岩心观察指明了页岩层系中主要发育介壳灰岩、夹介壳层页岩、含介壳页岩和纯页岩,各岩性中发育的孔缝类型多样,其中含介壳页岩和纯页岩中顺层微裂缝更加发育。为了进一步揭示页岩层系中发育的孔缝系统,开展了一维核磁实验来确定岩石中亲油孔缝和亲水孔缝相互间的比例关系(孔缝配置)。其中介壳灰岩和夹介壳层页岩的孔缝配置值>60%,指示油气在其中的运移将受到限制;受顺层微裂缝发育的影响,含介壳页岩和纯页岩的孔缝配置值<60%,说明其中的孔缝系统能成为较好的油气运移通道。受烃源供给能力和油气运移阻力的影响,在ω(TOC)>1.0%的岩石中,相较于孔缝配置值<60%,孔缝配置值>60%的岩石具有更多残留油量且可动性较好(S1在0.86~2.19 mg/g范围,均值为1.42 mg/g;OSI值在65.96~123.21 mg/g,均值为91.98 mg/g)。最终结合研究区有机质富集特征,阐明了研究区大安寨二亚段页岩油富集有利岩性组合。

The lithologic combinations in lacustrine shale strata in China are complex,and shale oil content varies significantly across different combinations.Identifying favorable lithologic combinations will assist in locating shale oil-rich zones.This study takes the shale strata of the second submember of the Da’anzhai Member(Da 2 submember)in Jurassic of the central Sichuan Basin as a case study.Using mineral composition analysis and core observation,four primary lithologies were identified:shell limestone,shale with interbedded shell layers,shell-bearing shale,and pure shale.Various types of pores and fractures develop in each lithology,with bedding-parallel microfractures being more developed in shell-bearing shale and pure shale.To further reveal the pore and fracture system in the shale strata,the proportion of oil-wet and water-wet pores and fractures in the rock(pore and fracture configuration)were determined through one-dimensional nuclear magnetic resonance(NMR)experi-ments.The pore and fracture configuration values of shell limestone and shale with interbedded shell layers exceeded 60%,indicating restricted oil and gas migration.Due to the development of bedding-parallel microfractures,the pore and fracture configuration values in shell-bearing shale and pure shale were less than 60%,indicating that these lithologies provided more favorable migration channels for oil and gas.Furthermore,influenced by the hydro-carbon supply capacity and the resistance in oil and gas migration,rocks with total organic carbon(TOC)con-tent greater than 1.0%as well as pore and fracture configuration values greater than 60%possessed more residual oil and exhibited better mobility than those with pore and fracture configuration values smaller than 60%(S1ranging from 0.86 to 2.19 mg/g,with an average of 1.42 mg/g;OSI values ranging from 65.96 to 123.21 mg/g,with an average of 91.98 mg/g).Finally,based on the characteristics of organic matter enrichment,the favorable lithologic combinations for shale oil enrichment in the Da 2 submember in the study area were clarified.