SEISMIC-ATTRIBUTE-DRIVEN ANALYSIS OF AN ULTRA-DEEP CARBONATE RESERVOIR: MIDDLE CAMBRIAN TO MIDDLE ORDOVICIAN STRATA, NORTH-CENTRAL TARIM BASIN, NORTHWEST CHINA

We use 3-D seismic data from an unnamed oilfield to characterize the internal architecture, structure, and porosity and permeability distributions in ultra-deep (. 6 km) Cambro-Ordovician carbonate reservoir strata deposited on a long-lived, regionally extensive carbonate platform in the Tarim Basin, northwest China. These strata comprise three dominant seismic facies: 1) tabular, layered units of subparallel to parallel, continuous reflections of medium to high amplitude (SPMAC facies), 2) mounds and lenses of chaotic to subparallel reflections of low to medium amplitude (CPLAD facies), which are widespread in certain stratigraphic levels, and 3) tabular, layered units of parallel, highly continuous reflections of very high amplitude (PVHAC facies). SPMAC facies represents low-energy lagoonal mudstones and wackestones, CPLAD facies represents bioclastic packstones, grainstones, and boundstones in reef–shoal complexes, and PVHAC facies represents alternating gypsum, dolomite, and wackestone–packstone lithologies in the lower part of the reservoir, and the top-reservoir contact between siliciclastic mudstones and packstones–grainstones. Seismic dip-magnitude and time–thickness maps show that CPLAD facies mounds and lenses are present in the middle to uppermost reservoir strata, and are associated with folded intervening and overlying SPMAC and PVHAC facies strata. Differential compaction folded these strata. A fourth, minor seismic facies, BPHAD facies, is recognized by parallel, laterally discontinuous, high-amplitude reflections that are stacked vertically and define rugby-ball-shaped beads, strings of beads, and subvertical cylinders. BPHAD facies is present in two settings. First, as multiple beads scattered within SPMAC and CPLAD facies strata. Here, we interpret them to represent widespread, small-scale reef–shoal complexes (, 500 m across) or collapsed karst caves. Second, BPHAD facies is present as strings of beads and subvertical cylinders near bends in strike-slip faults. We interpret these strings of beads and subvertical cylinders to be hypogene karst formed by hydrothermal fluids. This study predicts reservoir permeability and porosity with improved understanding of stratigraphic architecture in the Tarim Basin.