Abstract
A number of condensate reservoirs with high concentrations of H2S have been discovered in the deep dolomite reservoirs of the lower Ordovician Yingshan Formation (O1y) in the Tazhong Uplift, where the formation water has a high pH value. In the O1y reservoir, the concentrations of Mg2+ and SO42− in the formation water are higher than those in the upper Ordovician formation. The concentration of H2S in the condensate reservoirs and the concentration of Mg2+ in the formation water correlate well in the O1y reservoirs of the Tazhong Uplift, which indicates a presumed thermochemical sulfate reduction (TSR) origin of H2S according to the oxidation theory of contact ion-pairs (CIPs). Besides, the pH values of the formation water are positively correlated with the concentration of H2S in the condensate reservoirs, which may indicate that high pH might be another factor to promote and maintain TSR. Oil–source correlation of biomarkers in the sulfuretted condensates indicates the Cambrian source rocks could be the origin of condensates. The formation water in the condensate reservoirs of O1y is similar to that in the Cambrian; therefore, the TSR of sulfate-CIPs likely occurred in the Cambrian. High H2S-bearing condensates are mainly located near the No. 1 Fault and NE-SW strike-slip faults, which are the major migration pathway of deep fluids in the Tazhong Uplift. The redox between sulfate-CIPs and hydrocarbons is the generation mechanism of H2S in the deep dolomite condensate reservoirs of the Tazhong Uplift. This finding should be helpful to predict the fluid properties of deep dolomite reservoirs.
Highlights
Formation water has been conventionally taken as a kind of undesirable by-product in petroleum exploration and development, while it would be one of the most crucial factors in hydrocarbon generation, migration and accumulation as well as the interaction between organic and inorganic geo-fluids
We have reviewed more than 200 compositions of formation water in Tazhong Uplift, which were analyzed in the chemical laboratory of the Tarim Oilfield Company, in order to determine the origin of formation water in the condensate reservoirs
It has shown that the concentration of H2S in the O1y gas condensates has increased with the alkalinity of formation water (Fig. 7), which demonstrated that the pH of formation water may drop down in the process of forming contact ion-pairs (CIPs) from magnesium sulfate (He et al 2014)
Summary
Formation water has been conventionally taken as a kind of undesirable by-product in petroleum exploration and development, while it would be one of the most crucial factors in hydrocarbon generation, migration and accumulation as well as the interaction between organic and inorganic geo-fluids. Because the relative concentration of H2S is not a reliable reaction parameter to access the reaction intensity of TSR (Ma et al 2008; Jendu et al 2015), a systematic study on the genetic relationship between water and mineral composition of the Ordovician formation and high sulfur condensates would avail to clarify the formation mechanism of high sulfur condensate in the actual geological conditions These new insights could indicate the effect of ions in the formation water on the alteration of hydrocarbons in deep dolomite reservoirs, which would be useful to predict the fluid properties of deep stratum and promote the exploration of dolomite reservoirs. The difference in reservoir phases indicates that the distribution of condensates in the whole Tazhong Uplift roughly correlates with the fluid properties and burial depth and the dolomite reservoirs are correlated with NE-SW faults
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