Paleokarstification in carbonates of 4th Member of Middle Triassic Leikoupo Formation in northeastern Sichuan basin and its petroleum geological significance

The hydrocarbon breakthrough in the 4th member of the Leikoupo Formation shows important geological significance for petroleum exploration. Based on comprehensive analyses composed of outcrop and drill-core observations, thin-section petrographic examinations, well log analysis, fluid inclusion studies, and carbon, oxygen, strontium isotopic composition measurements, the authors divided paleokarst faces of the study area into six types, i.e., surface eluvium, speleothem composed of break-down clasts, fillings from underground rivers, crack of base rock, solution of base rock, and dissolution and replacement of base rock. According to hydrodynamic conditions, the paleokarstification can be divided in descending order into surface-karst zone, vertical vadose zone, horizontal phreatic zone and deep-karst zone. Using the impression method and based on the distribution characteristics of relict stratum, the authors restored palaeokarst geomorphology for the 4th Member of the Leikoupo Formation and presented more details of micro-geomorphology for Yuanba area. It is held that karst slope is the most advantageous geomorphic unit for the formation of reservoirs, with the most favorable place being middle-upper slope. Karst hammock is the most advantageous micro-geomorphic units for the formation of reservoirs. Paleokarstification plays an important role in the reservoir formation of 4th Member of Leikoupo Formation and its features control the distribution of favorable reservoirs in both vertical and planar directions.