Study Of Fossil Karst Reservoir In 4th-3rd Member Of The Leikoupo Formation In Longnvsi-Moxi Area In Central Sichuan Basin

Sichuan Basin is the biggest petroliferous basin in south China. Middle Triassic Leikoupo Formation is one of the main oil-bearing series of the central Sichuan Basin. The total gas production in central Sichuan basin is 10×108m3 per year, and 4×108m3 gas production each year is from the Leikoupo Formation. However, the present exploration is only limited to 1st sub-member of the 1st member of the Leikoupo Formation. The reservoir layer is a pinhole-algal clast dolomite. Study of others stratigraphic horizons in the Leikoupo Formation and others potential gas reservoir types, especially the Ancient Karst development interval at the top of the Leikoupo Formation are inadequately studied.This paper is discussing the fossil karst reservoir study at Longnvsi-Moxi area, by observing and analyzing the field outcrop,cores, thin section from the Leikoupo Formation in study area. By combining with previous achievements, using petrology, sedimentology, karstology, geomorphology , tectonic geology ,geochemisttry, it was divided diagenesis types, analysed characteristics and distribution of lithology, strata and sedimentary facies of the 4th-3rd member of the Leikoupo Formation in the study area. On this basis,we have divided the Ancient Karst types of Longnvsi- Moxi area, summarized each stage of ancient karstification, and made predictions about ancient karst reservoirs development according to ancient landscape restoration and the comparison with Longgang area.The ancient karst reservoirs according to diagenetic stage can be divided into three main types in the 4th-3rd member of the Leikoupo Formation in study area.There are: syngenetic dissolution, epidiagenetic dissolution and burial dissolution. Syngenetic dissolution was less developed, and was controlled by the secondary microgeomorphology of the carbonate platform. Epidiagenetic dissolution was controlled by lithology, sedimentary facies, tectonism and ancient landscape. Burial dissolution is divided into:①acid water dissolution,②hydrothermal dissolution, which controlled by migration pathway and tectonism.Reservoir in 4th-3rd member of the Leikoupo Formation in study area is mainly intercrystalline solution pores and intergranular dissolution pore, which was divided into three types:①structure-karst apertures type,②deep-buried karst apertures type,③many origin apertures type according to origins of the accumulate porosity in the rock.Longnvsi and Moxi area are located in the same tectonic-sedimentary setting and karst landform where is the karst slope that is developed the favorable karst zone. But because of lithologic restriction, there was no good karst apertures developed in Longnvsi area. There are good reservoir like Longgang area in Moxi area for the effect of epidiagenetic dissolution and late reformation of burial dissolution.Moxi-Tongnan zone which is in the southwest of study area and Luoduxi- Guangan zone are the favorable exploration zones in the study area.