Hydrogeochemistry of oil-field formation water in relation to diagenesis of reservoirs: A case study from Shahejie formation, Dongying Depression

The composition of formation waters in the Dongying Depression provide important clues about regional water-rock interactions. Chemical analysis (181 samples) of formation waters form the Shahejie Fm (58 wells) in the Niuzhuang oilfield suggest two main geochemical facies, the chloride-magnesium (Cl-Mg) and bicarbonate -sodium (HCO3-Na) facies (pH = 7.8 +/- 0.9) that have low salinity (TDS = 7.3 +/- 7.8 g/L) and occurs within source rocks, and the Cl-Ca facies (pH = 6.5 +/- 0.7) that has high salinity (TDS = 79.1 +/- 79.0 g/L) and occurs within clastic reservoirs. For the water within the clastic reservoirs, salinities and dissolved chloride concentrations (47.8 +/- 48.8 g/L) are attributed to halite dissolution. The broad systematic increase in dissolved Mg2+ and Ca2+, and decrease in pH with increasing salinity (Na+ concentration) strongly suggest that the formation water has been thermodynamically buffered by silicate-carbonate mineral assemblages. The Caexcess increase with Nadeficit suggests that the albitization of detrital plagioclase (anorthite) might have provided a internal source for Ca2+. The pH was mainly buffered by silicate minerals, which is indicated by the increase of acidity with Na+. This is also consistent with the externally buffered carbonate system indicated by the negative correlation of log(HCO3-) with log(Ca2+). The increasing acidity associated with the addition of Na+ might have been responsible for the abundant kaolinite and secondary porosity developed by K-feldspar dissolution. The contribution from halite dissolution to formation waters within the source rocks was relatively restricted. The positive correlation of log(HCO3-) with log(Ca2+) suggests an internally buffered carbonate system. The formation water might have been partly buffered by illitization, which is consistent with the negative correlation of Nadeficit with Caexcess.