--> Sedimentary Environment Differences of Ordovician Wufeng-Silurian Longmaxi Shale in China and Their Effects on Production – A Case Study of Weiyuan Shale Gas Field

2018 AAPG International Conference and Exhibition

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Sedimentary Environment Differences of Ordovician Wufeng-Silurian Longmaxi Shale in China and Their Effects on Production – A Case Study of Weiyuan Shale Gas Field

Abstract

Weiyuan Shale Gas Field is one of biggest shale gas field in China. Through practical exploration and development in recent years, it has found that prodution of horizontal well in different regions have significant differences. Finding the main geological factors that affect the horizontal well productivity of shale gas has become the focus of research. Based on the analysis of subdivision of graptolite biozone, organic geochemistry, mineral composition, elemental geochemistry of 10 wells in Weiyuan area and the seismic profiles, shale distribution, lithofacies types, redox conditions and sedimentary environment of different stages in the Late Ordovician Wufeng-Early Silurian Longmaxi shale have been Identified.It's concluded that: (1)the depositional stages in Weiyuan area can be divided into five stages: the late Katian, the Hirnantian ice stage, the early Rhuddanian, the late Rhuddanian, and the Aeronian. (2)From late Katian to Aeronian, the deposition rate of shale has accelerated and the sedimentary center has migrated eastwards.(3)Lithofacies distribution of shale at different periods has been identifid. The main lithofacies of the 5 stages are calcareous shale, shell limestone, silice shale,calcareous shale and argillaceous shale respectively.(4)Shale deposited in early Rhuddanian which is sedimented in anoxic and euxinic conditions is featured by high TOC, porosity and brittleness. The drilling length of horizontal well in this layer is the main geological factors controlling the high production of gas wells.(5)Considering the same engineering and technological factors, the deposition thickness of shale gas at the early stage of the Rhuddanian has a significant effect on shale gas production, and shale gas horizontal wells have a relatively high yield in this area. Based on the above research results, it is proposed that shale sedmentied in early stage of Rhuddanian is the target location of the shale gas horizental well , and development wells should be prioritized in areas where the thickness of this layer is large. The research results have effectively guided the exploration and development of shale gas in Weiyuan area.