Abstract: The exploration degree of buried hill limestone in Weixinan sag is low, and the exploration potential of remaining resources is large. Several buried hill oil fields or oil-bearing structures have been discovered around the No. 2 fault zone, which shows a good exploration prospect. In this paper, the development characteristics and karst structure identification of carbonate reservoir in No. 2 fault zone are systematically studied by using the data of drilling, logging, core, thin section and image logging, and the evolution and development model of karst reservoir in X structure is comprehensively analyzed according to the tectonic evolution and sedimentary background. The results show that the lithology of the buried hill of the X structure in the No. 2 fault zone is mainly limestone and lime dolomite, and the reservoir space is dominated by solution pores, caverns and fractures. The porosity varies from 0.01% to 35.88%, but the average value is only 4.30%. Taking well X-1 as an example, the supergene karst reservoir of X structure is divided into surface weathering zone, vertical seepage zone, horizontal subsurface flow zone and deep slow flow zone from top to bottom. The horizontal subsurface flow zone has the best karst fractures/caverns and is a high-quality reservoir development zone. The development degree of X tectonic fracture-cavern type limestone buried hill reservoir is mainly controlled by lithofacies, paleogeomorphology and late tectonic movement. The lithofacies determined the development degree of dissolution, and the tectonic movement caused differentiated denudation and formed a large number of fractures, which provided favorable percolation space for the later karst process. The paleogeomorphology controlled the relative position of the diving surface and thus controlled the development position of karst, and the high part of the karst slope was subjected to the strongest karst process. Based on three controlling factors, the three-element coupling reservoir development model of “lithology foundation building, fracture capacity expansion and karst reconstruction” is established.

Key words: Beibuwan basin, Weixinan sag, carbonate buried hill, reservoir characteristics, karstification, reservoir development model