47th Annual AAPG-SPE Eastern Section Joint Meeting

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From Outcrop To Reservoir, A Comprehensive Description To Lower Silurian Longmaxi Shale In Sichuan Basin, China


Technical recoverable resources for Lower Silurian Longmaxi shale (S1l) is estimated to be 286 tcf (EIA, 2016), showing prospective duplication of U.S shale gas revolution in China. Kerogen type, generation and accumulation patterns for Longmaxi shale have been reviewed by former researchers (Borjigin, 2017), which are conducive to resources evaluation. Whereas thorough appraisal to Longmaxi shale should be the foremost work for reservoir development from aspects of sedimentology, geochemistry, core sampling & description and well logging. In this paper, we proposed our comprehensive approach from outcrop investigation, in- situ measurements to reservoir evaluation for detail description of the Longmaxi shale. Firstly, lithology, lithfacies and sedimentary sequences have been reviewed through outcrops. Mark stratigraphy, carbonate rocks containing shell fossil have been found and two periods of transgression and regression can be discriminated. Then depositional framework established from carbonate platform to deep shelf during latter Ordovician to early Silurian. Organic rich black shale developed in the quiet deep water environment with relatively low clay content is primarily identified as the favorable target zone for development. Secondly, further petrophysical properties evaluation for different lithofacies through in-situ measurement have been conducted. Organic rich black shale has total organic content between 3.0%-8.8% with vitrinite reflectance ranged from 2.3%-3.6%. It also has relatively high porosity with mean value of 7.3%, and mean gas saturation of 85%. Then Lower Silurian Longmaxi Group can be classified into two zones and lower zone which is much sweater is finally identified as the target for horizontal drilling. Thirdly, relationship between field survey, in-situ measurement and well logging has been connected and eletrofacies zonation has been established. The targeted lower zone has much higher natural gamma, higher resistivity, high interval acoustic transit time, lower density and thorium to uranium ratio. Finally, sedimentary framework of Lower Silurian Longmaxi Group and detailed reservoir architecture have been established, and the sweat and the target zone has also been identified. Pilot drilling shows that horizontal wells with high penetration ratio of the identified target zone have the double outputs which reveals the meaning of our works.