NEW INSIGHTS INTO FAR EASTERN SHALE PLAYS; THE SHALE GAS ACCUMULATION OF THE TRIASSIC YANCHANG FORMATION IN THE ORDOS BASIN, CHINA

The Chang 7 shale member of the Triassic Yanchang Formation in the Ordos Basin is an important oil source, and new exploration suggests it is also a favorable prospect for shale gas. This study investigated the shale gas potential of the Chang 7 member and associated lithofacies using 279 well logging data and laboratory analysis of 347 core samples from 18 wells. We identified four primary lithofacies within the Chang 7 member. The highest source quality lithofacies is a dark shale composed of clay minerals, quartz, and feldspar, with abundant organic matter. Other lithofacies include a dark gray fossiliferous mudstone lithofacies, a gray carbonaceous silty mudstone lithofacies, and a gray argillaceous siltstone lithofacies. The lithofacies can alternate at a centimeter scale, and cannot be separated in petrophysical well log data.

The total organic carbon (TOC) in the dark shale lithofacies ranges from 0.65-21.0%; 80.7% of the formation is estimated to be >2% TOC. Vitrinite reflectance (Ro) is between 0.7-1.13%, and most of the samples are in the lower mature or mature stage for gas. Mineral compositions of the dark shale are quartz (27.7%), plagioclase (18.4%), potassium feldspar (7.8)%, pyrite (1.3%), siderite (9.6%), calcite (1.3%), ferroan dolomite (0.7%), and clay minerals (42.16). Multiple porosity types were observed under scanning electron microscopy (SEM) in the Chang 7 member. Most pore types are intergranular. Intercrystalline pores are associated with authigenic mineralization, and the member also contains dissolution pores. Pore diameters range between 1~10µm.

Air bubbles were observed along bedding planes and at the grain level in core tests. We suggest that the dry gas output can reach 1000-3000 cubic meter/day. There is an observed relationship between gas content and lithofacies, as gas contents are higher in the gray carbonaceous clastic silty mudstone lithofacies and the gray argillaceous siltstone lithofacies. Although the adsorption volume records a correlation with carbon content and maturity, the total volume of adsorbed gas is not high. This study suggests the need for further shale gas exploration of the Chang 7 member of Yanchang Formation in the Ordos Basin.