Eogenetic karst and its control on reservoirs in the Ordovician Majiagou Formation, eastern Sulige gas field, Ordos Basin, NW China
(a) Well T42, core, 3 405.78 m, M52 sub-member, brownish gray micrite dolomite, the lower part is black argillaceous dolomite; (b) Well T42, core, 3 405.90 m, M52 sub-member, dark gray argillaceous dolomite with black transgression argillaceous sediments; (c) Well T42, core, 3 406.02 m, M52 sub-member, black argillaceous dolomite, with thin layers of brownish gray micrite dolomite; (d) Well T42, core, 3 406.12 m, M52 sub-member, exposure surface, with black argillaceous dolomite above and brownish gray limy breccia dolomite with dedolomitization below; (e) Well T42, core, 3 406.22 m, M52 sub-member, brown gray gray-breccia dolomite, dedolomitization; (f) Well T42, core, 3 406.28 m, M52 sub-member, brown gray limy micrite dolomite, dedolomitization; (g) Well T42, core, 3 406.48 m, M52 sub-member, brownish gray to gray-breccia dolomite, dedolomitization; (h) Well T42, core, 3406.66 m, M52 sub-member, brownish gray to gray breccia dolomite, de-dolomitized; (i) Well T14, core, 3612.48 m, M54 sub-member, brownish gray algal stromatolite, with gravity fault and collapse deformation; (j) Well T14, core, 3 612.66 m, M54 sub-member, exposure surface, with grayish black argillaceous dolomite above and de-dolomitized brownish gray limy breccia below, and high-frequency overlying transgression sediments filling between breccias; (k) Well T14, core, 3612.80 m, M54 sub-member, brownish gray limy dolomite with spongy dissolution; (l) Well T14, core, 3 612.96 m, M54 sub-member, dark gray limy dolomite with spongy dissolution, dedolomitization, horizontal karst caves fully filled with seepage fillings and calcite; (m) Well T14, core, 3 613.06 m, M54 sub-member, de-dolomitized light gray limy dolomite, dedolomitization; (n) Well T14, core, 3 613.25 m, M54 sub-member, de-dolomitized gray limy dolomite, dedolomitization.
