Geochemical constraint on the origin of the multi-mineralogic carbonate cements in the subsurface Middle Jurassic sandstones, Central Sinai, Egypt

Origin and Distribution of Carbonate Cement in Tight Sandstones: The Upper Triassic Yanchang Formation Chang 8 Oil Layer in West Ordos Basin, China

“…In contrast, reprecipitation and recrystallization during burial (increase in burial and temperature) would lead to a further negative shift in 18 O values [49]. The high 18 Odepleted signature, negative 13 C values, and high Fe and Mg contents suggest that the late replacive carbonate cement (Fe-calcite and Fe-dolomite) including Fe-dolomite and Fecalcite formed in burial conditions via the replacement of early diagenetic carbonate cement by the influence of deep formation water [45]. The positive 13 C values of some dolomite cement in Chang 8 sandstones are compatible with early marine carbonates but not the oxygen isotopes (Figure 8; [27]).…”

“…The eogenetic cement may have supported the framework grains and prevented extensive compaction [42], resulting in the floating grain texture and high volume of high minuscement porosity [43]. The pore-filling cement is suggested to have precipitated before significant compaction and thus support eogenetic origin [44,45]; however, the carbonate cement replacing framework grains may also precipitate after significant compaction. They display bright orange-red and yellow luminescence patterns under cathodoluminescence analysis (Figures 7(e) and 7(f)).…”

“…It is difficult to define precisely the timing of the cementation processes [45,50]. Two diagenetic stages of carbonate cement are recognized according to petrographic and textural relationships and elemental and stable isotopic composition: the early diagenetic stage in shallow depth, represented by porefilling and grain-replacing calcite and dolomite, and the late mesogenetic deep burial diagenetic stage, represented by ferroan calcite and ferroan dolomite replacing framework grains and precipitating in intergranular pores.…”

“…The presence of carbonate cement in the loosely packed framework grains indicates that cementation commenced shortly after deposition [45]. The slightly negative 13 C values of the dolomite and calcite cement would indicate its precipitation in slightly reducing conditions with minor contribution of organic carbon derived from soil-derived CO 2 [45].…”

“…The slightly negative 13 C values of the dolomite and calcite cement would indicate its precipitation in slightly reducing conditions with minor contribution of organic carbon derived from soil-derived CO 2 [45]. Indeed, soil CO 2 could also give the depleted 13 C values, and it depends on how much soil CO 2 was incorporated into the diagenetic fluids.…”