Carbonate cements in Eocene turbidite sandstones, Dongying Depression, Bohai Bay Basin; origin, distribution, and effect on reservoir properties

Diagenetic carbonate cements occur throughout Eocene turbidite lithic arkose in Niuzhuang sag, eastern China, displaying as sporadical siderite, dolomite, calcite 1, and massive ankerite and calcite 2. Petrographic and geochemical investigations suggest that the nonferroan calcite 1 (delta (super 13) C (sub carbonate (carb)) +3.1 ppm to +4.7 ppm Vienna Peedee belemnite [VPDB]; delta (super 18) O (sub carb) -12.5 ppm to -10.5 ppm VPDB) was the earliest carbonate cement that was followed by Ak (delta (super 13) C (sub carb) +0.2 ppm to +5.1 ppm VPDB; delta (super 18) O (sub carb) -12.8 ppm to -10 ppm VPDB) and ferroan calcite 2 (delta (super 13) C (sub carb) +2 ppm to +3.1 ppm VPDB; delta (super 18) O (sub carb) -12.6 ppm to -15.8 ppm VPDB). The homogenization temperatures (74.3 degrees C-105.8 degrees C, 105.6 degrees C-130.4 degrees C, and 119.7 degrees C-144.6 degrees C, respectively) reflect the pattern of increasing temperature with progressive burial. The delta (super 13) C (sub carb) values suggest that calcite 1 and calcite 2 were mainly derived from dissolution of carbonates in calcareous shales (delta (super 13) C (sub carb) +3.5 ppm to +6.3 ppm VPDB), with minor contributions from organic matter. The delta (super 13) C (sub carb) values of ankerite documented some contributions from magmatic carbon. Ankerite and calcite 2 were restricted under the top seal (geochemical barrier) of the overpressure compartment, and the highly cemented zones occur mainly along faults. Carbonate cementation seems to negatively impact the reservoir quality when it exceeds 10%.