Eogenetic karst and its control on reservoirs in the Ordovician Majiagou Formation, eastern Sulige gas field, Ordos Basin, NW China

XIE Kang,TAN Xiucheng,FENG Min,WANG Baobao,ZHONG Shoukang,YANG Mengying,NIE Wancai,QIAO Zhanfeng,ZENG Wei

Fig. 3. Petrological characteristics of reservoir rocks in the M54-M51 sub-members of the eastern Sulige gas field. (a) Well S25, core, 3347.95 m, M53 sub-member, gray micrite dolomite, homogeneous mass; (b) Well T51, core, 3429.66 m, M51 sub-member, gray micrite dolomite with moldic pore, the moldic pores are isolated, some of them are enlarged into spongy dissolution; (c) Well SD47-42, core, 3307.87 m, M54 sub-member, brown-gray breccia dolomite, the bedrock is micrite dolomite with moldic pore, and the moldic pores are isolated; (d) Well T36, core, 3229.60 m, M51 sub-member, left: brown-gray stromatolite, the dark layer is dark-gray bacteria-rich micrite dolomite, the bright layer is brown-gray micrite dolomite, right: brown-gray laminated dolomite; (e) Well S25, 3347.95 m, M53 sub-member, lamellar micrite dolomite, the dark layer is argillaceous micrite dolomite, the light layer is powder crystal dolomite, ordinary thin section, single polarized light; (f) Well T6, 3587.78 m, M54 sub-member, micrite dolomite, with intergranular dissolution micropores developed in the bedrock, and karrens partially filled with dolomite breccias and seepage fillings, single-polarized light; (g) Well SD47-42, 3307.87 m, M54 sub-member, micrite dolomite with moldic pore, the moldic pores are partially filled with seepage sand and quartz, single polarized light; (h) Well T2, 3384.86 m, M52 sub-member, micrite dolomite with moldic pore, the moldic pores are isolated, in approximately square shape, and partially filled with calcite, argillaceous seepage sand etc, single polarized light; (i) Well S25, 3355.38 m, M53 sub-member, sparry dolarenite, with intergranular dissolution pores and freshwater dolomite cement, single polarized light; (j) Well T36, 3390.08 m, M54 sub-member, powder crystal dolomite, with grain shadow, intercrystalline (dissolution) pores, and calcite cement, single polarized light; (k) Well T51, 3429.37 m, M51 sub-member, sparry oolitic dolomite, with intergranular pores partially filled with micritized freshwater dolomite cement and seepage silt, single polarized light; (l) Well T51, 3427.54 m, M51 sub-member, micrite oolitic dolomite, with visible residual oolitic structure, intergranular pores partially or completely filled by seepage sand etc, single polarized light; (m) Well Zh97, 3448.44 m, M54 sub-member, sparry dolarenite, with intragranular dissolution pores developed in grains, single polarized light; (n) Well T14, 3615.05 m, M54 sub-member, algal bound clot dolomite, with unfilled framework pores, single polarized light; (o) Well T42, 3407.40 m, M52 sub-member, laminated dolomite, with horizontal laminae and stromatactis fully filled with fine to medium crystalline calcite and other minerals, single polarized light; (P) Well Zh96, 3382.85 m, M51 sub-member, algal bound dolarenite, with calcite cement filling between grains, single polarized light.