Impact of Differential Densification on the Pore Structure of Tight Gas Sandstone: Evidence from the Permian Shihezi and Shanxi Formations, Eastern Sulige Gas Field, Ordos Basin, China

Figure 7

Photomicrographs showing typical minerals and pores of representative samples for each lithologic mode. (a) High content of plastically deformed cuttings, which fill in particles after compaction, SD24-55, 2948.97 m. (b) Sandstone rich in rigidly deformed cuttings, and quartz overgrowth and calcite cements can be occasionally seen, SD24-55, 2980.43 m. (c) Tightly cemented calcite and well-preserved intergranular pores, part of the quartz developed overgrowth, SD24-55 and 2798.74 m. (d) High quartz content with common quartz overgrowth and minor calcite cements, SD24-55, 3014.56 m. (e) Calcite crystal stock cementation, Z7, 3025.31 m. (f) Chlorite rim can be seen. Authigenic quartz is generally developed in intergranular pores, with minor dissolved pores, SD24-55, 2974.32 m. (g) Low matrix content. Quartz developed overgrowth. Ferrocalcite was tightly cemented during the late stage of diagenesis, Z7, 2914.82 m. (h) Chlorite is weakly cemented, and dissolution occurred. Intergranular pores and kaolinite intercrystalline pores formed due to feldspar dissolution, Z10, 3108.926 m (Q = quartz; F = feldspar; Cln = chalcedony; Qa = authigenic quartz; Qo = quartz overgrowth; M = matrix; Ca = carbonate cementation; Cln = chalcedony; K = kaolinite; Ms = muscovite; Mqs = mica quartz schist rock debris; Sp = sericite phyllite rock debris; Ph = phyllite rock debris; Slt = siltstone rock debris; Qte = quartzite rock debris; Mqr = mica quartzite rock debris; φ = pore).

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