Shale gas reservoirs of the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation,Dazu area,Sichuan Basin:Characteristics and distribution
Deep shale from the Upper Ordovician Wufeng Formation to the Lower Silurian Longmaxi Formation(Wufeng-Longmaxi forma-tions),with burial depth ranging from 3,500 m to 4,500 m,has emerged as the main domain for marine shale gas exploration in Sichuan Basin.Dazu area,southern margin of central Sichuan uplift(central basin),is characterized by gentle structures without any faults.But shale reservoirs in this area exhibit strong heterogeneity both vertically and horizontally,leading to great difference between production performance and avail-able reservoir evaluation.So,a fine stratigraphic correlation was performed on the deep shale based on data of well logging and core analysis to-gether with paleogeomorphologic research.A method of reservoir evaluation was built up for massive and continuous sweet spots.Both vertical and horizontal distribution was made clear for the shale reservoirs.Results indicate that(i)the sedimentary thickness from Wufeng Formation to the first submember of the first member of Longmaxi Formation(Long11 submember)is controlled by three paleogeomorphologic units,indicat-ing thicker in the south than in the north.And the depocenter also from Wufeng Formation to Long11 submember is located in the north of unit Ⅲ;(ii)reservoir distribution differs from stratigraphic extension.And the total thickness of Class Ⅰ reservoirs from Wufeng Formation to Long11 submember within unit Ⅱ is larger than that within units Ⅰ and Ⅲ;and(iii)unit Ⅱ is the most favorable belt in Dazu area,while unit I with devel-opment potential as another belt of productivity replacement.The above results have enriched understandings on both organic-rich shale thick-ness and distribution of quality reservoirs in Dazu area from Late Ordovician to Early Silurian,and provide a new tool for reservoir evaluation and ideas to optimize exploration targets for the efficient shale-gas development in Wufeng-Longmaxi formations,southern margin of central Sichuan uplift.
Sichuan BasinDazu areaDeep shale gasWufeng-Longmaxi formationsPaleogeomorphologic unitEvaluation methodClass I reservoirReservoir distribution
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