Characterization of favorable lithofacies in tight sandstone reservoirs and its significance for gas exploration and exploitation: A case study of the 2nd Member of Triassic Xujiahe Formation in the Xinchang area, Sichuan Basin

LIU Junlong,LIU Zhongqun,XIAO Kaihua,HUANG Yanqing,JIN Wujun

Table 1 Classification of lithofacies in the T3x2 Member of Xinchang tectonic zone.

Lithofacies Code Features Geologic interpretation Physical properties Poro- sity/% Permeability/10-3 μm2 Multiple layer medium-coarse grain sandstone CMpp Medium-coarse grained sandstone, with parallel bedding or trough cross bedding, and high-density crisp cake-like bedding seam of certain opening Occurring at the bottom of braided channel, indicating stronger hydrodynamics 2.5-6.3 (4.1) 2.3-33.2 (15.4) Parallel bedding medium-coarse grain sandstone CMpl Medium-coarse grained sandstone, with well-developed parallel beddings, not open along bedding seam Occurring at the bottom of braided channel, indicating moderate-strong hydrodynamics 2.4-9.0 (5.6) 0.01-757.00 (24.50) Massive bedding medium-coarse grain sandstone CMms Medium-coarse grained sandstone, with massive bedding, generally thicker single layers Appearing in the middle-lower sedimentary sequence of braided channel, indicating rapid accumulation process and moderate hydrodynamics 1.0-12.0 (5.1) 0.002-15.000 (0.400) Mud gravel- bearing medium- coarse grain sandstone CMmc Medium-coarse grained sandstone, with irregular mud gravels Generally turning up at the bottom-most of braided channel, indicating channel scouring and retention sedimentation at the bottom 0.4-6.8 (2.1) 0.001-7.000 (0.800) Charcoal-bearing medium-coarse grain sandstone CMcr Medium-coarse grained sandstone, with thin coaly debris Generally occurring at the bottom-most of braided channel, indicating retention sedimentation at the bottom, formed by scouring of river to swamp 1.7-12.1 (3.8) 0.003-16.800 (0.500) Inclined bedding medium-coarse grain sandstone CMcb Medium-coarse grained sandstone, with inclined bedding Appearing in the middle of the channel sedimentary sequence, indicating moderate- strong hydrodynamics 2.2-11.8 (4.0) 0.003-7.800 (0.200) Inclined bedding fine grain sandstone Fcb Fine grained sandstone, with inclined bedding Coming up in the middle-upper part of the channel sedimentary sequence, indicating moderate hydrodynamics 0.6-5.9 (2.5) 0.001-2.800 (0.100) Coaly mudstone MScl Thin layer, with apparent carbonization Generally turning up at the top of a cycle, swamp facies Wavy bedding siltstone Swb Siltstone, wavy sand ripple bedding Occurring in the middle-upper part of the channel sedimentary sequence, indicating weak-moderate hydrodynamics 0.3-2.7 (1.0) 0.002-3.400 (0.100) Carbonaceous mudstone MScr Mudstone, with carbonaceous debris Appearing at the top of a cycle, flood plain facies Calcarinate massive bedding medium-coarse grain sandstone CMmst Medium-coarse grained sandstone, massive bedding; blisters violently when adding acid Occurring in the middle-lower sedimentary sequence of braided channel, indicating moderate hydrodynamics 1.7-2.5 (2.0) 0.02-0.10 (0.06) Calcarinate inclined bedding medium-coarse grain sandstone CMcbt Medium-coarse grained sandstone, inclined bedding; blisters violently when adding acid Turning up in the middle of the channel sedimentary sequence, indicating moderate-strong hydrodynamics 0.5-4.3 (2.1) 0.000 3- 2.000 0 (0.100 0)