Differences between reservoirs in the intra-platform and platform margin reef-shoal complexes of the Upper Ordovician Lianglitag Formation in the Tazhong oil field, NW China, and corresponding exploration strategies

The Upper Ordovician Lianglitag Formation in the Tazhong oil field, NW China, is dominantly composed of shallow marine carbonates, and it can be subdivided into five members lithologically. Unclear understanding of the reservoir evolution in the platform margin and intra-platform reef-shoal complexes resulted in difficulty in locating reservoirs and poor drilling success in the intra-platform. By comparing the basic characteristics, diagenesis, and porosity evolution of the two types of reservoirs, this study established their development patterns and proposed corresponding exploration strategies based on comprehensive analysis of core, thin-section, well-logging, three-dimensional seismic, and geochemistry data. The major difference between the platform margin and intra-platform reservoirs lied in their pore types. The platform margin reservoirs had fabric-selective intragranular dissolution pores, intergranular dissolution pores, moldic pores, and non-fabric-selective karst fractures and caves, and tectonic fractures, whereas the intra-platform ones only had tectonic fractures, intercrystalline dissolution pores, and intergranular vugs. In terms of distribution, the platform margin reservoirs were mainly distributed in the parasequence top and landform highlands of the highstand system tract in Members 3 to 1, appearing in two to six cycles, with large single layer and accumulative thicknesses and a quasi-bedded continuous distribution in the lateral. However, the distribution of the intra-platform ones was not obviously correlated with sedimentary facies and paleogeomorphology, with smaller single layer and accumulative thicknesses and poorer lateral continuity. The evolution and distribution of high-quality reservoirs in the platform margin were jointly controlled by eogenetic marine diagenetic and meteoric dissolution including karst, burial dissolution, and tectonic fractures. In comparison, eogenetic marine diagenetic and meteoric dissolution in the intra-platform reef-shoal complexes were weak, accompanied by persistent cementation. The favorable reservoirs were primarily controlled by fractures and later burial dissolution. Hence, for the exploration of the platform margin reef-shoal complex reservoirs, the priority should be given to the areas with the superimposition of eogenetic marine diagenetic and meteoric dissolution, burial dissolution, and tectonic fracturing. For the exploration of intra-platform reef-shoal complex reservoirs, more attention should be paid to areas with overlapped fractures and burial dissolution, especially where Hercynian strike-slip faults and early fractures were superimposed. Less attention should be paid to the areas with eogenetic marine diagenetic and meteoric dissolution.