A development model and petroleum exploration of karst reservoirs of Ordovician Yingshan Formation in the northern slope of Tazhong palaeouplift

In the northern slope of the Tazhong palaeouplift,large-scale high-quality reservoirs with karst fractures and cavities are well developed along the unconformity of the top part of the Ordovician Yingshan Formation,where the cumulatively proved oil in place is nearly 200 million tons.The reservoir space of these karst reservoirs is dominated by large fractures and vugs,penetrating through a depth of 200m below the unconformity.These fractures and vugs are characterized by the superimposition of fractures and vugs with heterogeneous change and by a banded distribution.These reservoirs are not only related to ancient karst topography,but also controlled by many other integrated factors,such as multi-origins,multi-phase diagenetic dissolution,fracture cracks,superimposition of burial dissolution.The fracture and fracture network is a major percolation channel of karst water.And the unconformity karst controls the horizontal distribution scale of reservoirs,and burial dissolution is a key for the optimization of reservoirs.By detailed investigations on these main controlling factors,a model for the superimposed development and evolution of reservoirs was established and a comprehensive forecast for favorable zones of reservoir development was made as well.It was pointed out that it was necessary to pay attention to differences in reservoir types and main factors between different well blocks,and finally an optimal selection of main controlling factors was made for exploration guidance.