Natural gas exploration prospect in the Upper Paleozoic strata, NW Sichuan Basin

Abstract

Progress of natural gas exploration has recently been made in the Upper Paleozoic marine-facies reservoirs in the NW Sichuan Basin. It has been proved that the Middle Permian Qixia Fm pore-type dolomite reservoirs may be distributed continuously in a large area, besides, for the first time favorable gas-bearing Devonian dolomite reservoirs in great thickness were discovered in the downhole there. Recent research achievements indicate that: (1) The Lower Cambrian quality source rocks in huge thickness were found in the study sedimentary area with advantageous hydrocarbon source conditions. (2) Influenced by the Caledonian movement, the whole NW Sichuan Basin was lifted up to form a massive box-like NS-trending plunging uplift low steep in the west but high sluggish in the east, laying a robust foundation for the inheritance of Devonian, Carboniferous, and Permian platform margin reef large-scale sedimentation and dolomitization. (3) In the Upper Paleozoic strata, there were multiple planes of unconformity and multi-stage supergene karst occurred, improving the preservation and seeping capacity of reservoirs. (4) Tectonic deformations resulted from the nappe structure in the Longmenshan Mountain provided good hydrocarbon source fractures and sealing conditions for oil and gas reservoirs, which is conductive to the formation of a giant gas pool. It is concluded that the back-thrust anticline belt east to Hidden fault I in this study area has favorable conditions for large-scale structural–stratigraphic and structural–lithologic combination traps. As suggested in this study, relatively strong dolomitization was found in the Qixia Fm in the Shejianhe River–Zhongba higher palaeogeomorphic deposit area, so the most favorable exploration target of the Qixia Fm will be around the central axis of Caledonian paleo-uplift to the east ridgeline, and the most favorable prospect of the Upper Palaeozoic will be the central axis to the west of Hidden fault I where three gas bearing reservoirs will be possibly explored at the same time.