Lithofacies characteristics and evaluation of shale oil source and reservoir in Da 'anzhai member of lower Jurassic in northeast Sichuan: Case study from section of Tieshan Jinwo and Liangping Fuluzhen

Key words: 
• northeast Sichuan /
• Sichuan Basin /
• Da'anzhai Member /
• Jurassic /
• shale layer /
• sedimentary facies /
• shale oil /
• favorable intervals

Abstract: [Objective] To understand the sedimentary characteristics of shell limestone-shale mixed strata in the Da’anzhai Member of Sichuan Basin and its influence on favorable shale oil horizon, field profiles in Dazhou Tieshan Jinwo and Liangping Fuluzhen of the northeast Sichuan Basin were studied. [Methods] The lithology and sequence, source and reservoir quality, and favorable strata of the Da’anzhai Member were evaluated in detail by using rock thin sections, X-ray diffraction (XRD) whole rock and organic geochemical analyses. [Results] The results show that: 1) The Da’anzhai Member is a set of limestone-shale mixed strata, and different lithologies are superimposed in an orderly way to form a variety of upward shallower sequences. From shallow lake to semi-deep lake-deep lake, six kinds of upward shallower decimeter to meter lithofacies sequences are identified: ①shale-shell shale, ②shale-thin shell limestone, ③shale-medium shell limestone, ④shale-nodular micrite, ⑤thin shale-shell limestone-crystalline limestone, and ⑥shell limestone-siltstone/fine sandstone. 2) The quality of source and reservoir varies greatly among different lithologic sequences. Black shales in sequence 1–3 are developed with the best source quality. The organic carbon content of black shales in a single sequence shows a trend of higher and lower organic carbon content, and total organic carbon (TOC) gradually decreases with the increase of shell limestone. Different lithologic properties vary greatly. The porosity of shale is higher than that of limestone and siltstone, but the clay content is high, the pore size is small, and the seepage capacity is poor. Limestone easily develops joint fractures, and the density of joints decreases exponentially with the increase of limestone thickness. Therefore, Sequence 2–4 developed middle and thin layer limestone has better reservoir performance. 3) The field oil seedling shows that the shale oil reservoir of the Da’anzhai Member is characterized by the separation of source and reservoir and thin high-quality reservoir. The shale oil seepages in the field are mainly distributed near the joints of the medium–thin layer shell limestone. The favorable reservoir is primarily controlled by the sedimentary conditions, micro-fractures, and configuration of source and reservoir. [Conclusions] Sequences 2 and 3 have the best source and reservoir configuration conditions, which are favorable intervals for shale oil.