Development Patterns And Controlling Factors Of Karst Reservoir Of The Maokou Formation In The Sichuan Basin

The end of the Middle Permian was a critical period of regional unconformity formation in the Sichuan Basin.A series of small to medium-sized gas reservoirs have been discovered around the Luzhou paleohigh,southern Sichuan Basin,where the Maokou Formation karst reservoir is developed.In recent years,with the advancement of exploration process,karst reservoirs of Maokou Formation have also been found in the central and northern Sichuan Basin.Therefore,it is necessary to study on the development patterns and controlling factors of the Maokou paleokarst and the related reservoirs from the basin scale based on available data,which could provide reference for the future natural gas exploration.Based on core and outcrop observations,combined with well drilling,well logging and seismic data and via carbonate sedimentologic,petrologic,sequence stratigraphic and karst analytical methodologies,this thesis identifies the exposed unconformity characteristics of the sequence stratigraphic surfaces in the Maokou Formation,analyzes the paleotectonic and paleogeomorphological conditions of paleokarst development,classifies the karst development types,clarifies the karst development characteristics and their controlling factors of every type,and establishes the corresponding karst reservoir development patterns.The following conclusions are obtained:(1)At the end of the Middle Permian,the paleomorphology of the Maokou Formation in the Sichuan Basin was generally gentle,with a topographic relief <150 m and an average slope of about0.02°.In the meantime,some specific geomorphic units were developed in local areas,including the Luzhou inherited paleohigh in southern Sichuan Basin,the rift formed by basement fault activity in northern Sichuan Basin,and the basaltic uplift formed by the eruption of Emeishan mantle plume along with the adjacent erosional valley at the top Maokou Formation in western and southwestern Sichuan Basin.Four types of paleokarst can be classified according to the paleogeomorphological characteristics and geographical settings: continental karst around the Luzhou paleohigh in southern Sichuan Basin,as well as plain-like,coastal and erosional valley induced karst in the immense karst basin.(2)The development and distribution of the paleokarst features of the Maokou Formation around the Luzhou paleohigh were controlled by paleotopographic relief,presence of paleofaults and shoal development,which were mainly distributed on the edge of the karst highland and in the western and upper eastern karst slope laterally,and in the shoals of the second and third member(TR2 sequence)vertically.On the other hand,near the paleofaults,karst features have been obviously enhanced with the faultcontrolled caves relatively large in scale.Lateral distribution of gas reservoirs’ performance data indicates that the Maokou karst reservoirs are concentrated in the fault zones and the shoal development area around the paleohigh.(3)The topography of the karst basin area was gentle,resulting in the development of vertical karst at vadose zone above the water table,and the frequent chemical precipitation below due to the groundwater flow stagnation along with the underdeveloped phreatic caves.The fluctuation of water table regulated the development of vertical karst features,which were enlarged since the vadose zone expanded when the water table falling,and were submerged by phreatic water and filled with calcites when rising.Core and well logging data reveal that the vertical karst fissures and caves at the top of the Maokou Formation in the central Sichuan Basin and the Yuanba area of northern Sichuan Basin have been mostly occupied by calcite cements,resulting in relatively poor porosity and permeability.Besides,confirmed by the existing boreholes,karstification has been enhanced in the paleofault zones and the erosional valley zones adjacent to the basalts in the southwestern Sichuan Basin,where the karst reservoir is developed.(4)The most favorable zones for the development of the Maokou Formation karst reservoir in the Sichuan Basin are the shoal development area and the paleofault zones in the surrounding area of LuzhouHechuan-Guangan paleohigh.Also,the local paleofault zones in the central and eastern Sichuan Basin and the erosional valley zones adjacent to the Emeishan basalts in the southwestern and western Sichuan Basin are positive to the reservoir development.