Characteristics, formation and evolution of pre-salt dolomite reservoirs in the fifth member of the Ordovician Majiagou Formation, mid-east Ordos Basin, NW China

FU Siyi,ZHANG Chenggong,CHEN Hongde,CHEN Anqing,ZHAO Junxing,SU Zhongtang,YANG Shuai,WANG Guo,MI Wentian

Fig. 3. Pore types and diagenesis of pre-salt dolomite in the mid-eastern Ordos Basin. (a) Well J-12,3 668.45 m, M58 sub-member, dissolution pores on the surface of core; (b) Well J-6, 3 583.10 m, M510 sub-member, gypsum moldic pores, cast thin section; (c) Well J-12, 3 647.45 m, M58 sub-member, dissolution vugs on core surface filled with salt, core photo; (d) Well J-12, 3 647.32 m, M58 sub-member, dissolved pores occurring along the fractures in band shape, cast thin section; (e) Well J-6, 3 556.90 m, M59 sub-member, dolomite formed by recrystallization cut by expending fracture, cast thin section; (f) Well J-2, 3 590.10 m, M57 sub-member, gypsum between grains is dissolved, the fractures and pores are totally or partially filled with organic matter, cast thin section; (g) Well J-6, 3 583.10 m, M57 sub-member, anhydrite is replaced by calcite, with pyrite developing around, ordinary thin section; (h) Well J-6, 3 615.84 m, M510 sub-member, fracture totally filled with anhydrite, cast thin section; (i) Well J-12, 3 642.41 m, M57 sub-member, recrystallization giving rise to big dolomite patches, resulting in the loss of fractures and pores, casting thin section.