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

Abstract

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.

An Overview of the Differential Carbonate Reservoir Characteristic and Exploitation Challenge in the Tarim Basin (NW China)

“…Generally, the high-energy facies in the Ordovician period have been assumed as the basic condition for the porosity development in the ancient reservoirs [16,[22][23][24]. This agrees with the fact that high oil/gas production wells in the reef-shoal facies are much more than that in the carbonate intraplatform [8,16].…”

“…The residual primary porosity is less than 0.5% in the reef-shoal reservoirs. It has beenargued that the Ordovician platform margin is favorable for the penecontemporaneous dissolution [16,[22][23][24], whereas more borehole data have shown that most reef-shoals along the Ordovician carbonate platform margins present tight reservoirs with low-porosity (<2%) and low-permeability (<0.2 mD). Further studies suggest that the high matrix porosity is related to transitional karstification by the thrust uplift in the central Tarim Basin [8,17,26].…”