Marine carbonate reservoirs formed in evaporite sequences in sedimentary basins: A review and new model of epeiric basin-scale moldic reservoirs

The development of evaporite-carbonate assemblages is a common sedimentary process in restricted and evaporitic marine environments. These assemblages provide important reservoir spaces for oil and gas. To establish a systematic understanding of, and scientific basis for, oil and gas exploration, this paper reviews the major genetic environments of evaporite-carbonate assemblages and associated hydrocarbon reservoirs. Based on the review, carbonate reservoir-forming marine evaporite sequence worldwide can be categorized into four major settings: marginal-marine sabkhas, marginal-marine evaporative lagoons, coastal salinas and regional evaporite basins. In addition, a new type of reservoir that developed in the Ordos Basin of China during the Ordovician greenhouse is proposed based on a synthesis of new and existing chemical and physical data. This new model is identified in the supra-salt reservoir in the fifth Member of the Middle Ordovician Majiagou Formation in the Ordos Basin and is termed an epeiric basin-scale moldic reservoirs. In this newly identified type of dolomite reservoir, irregular-circular moldic pores and vugs that are uniformly distributed in micritic dolomite matrix formed high-quality reservoir spaces, examples of which are rare worldwide to date. The primary evaporitic mineral in these molds is disseminated halite crystal, which formed in a restricted and evaporitic epeiric-basin environment. The selective dissolution of disseminated halite crystals and enlargement of its surrounding rocks in the upper part of the sedimentary cycle by meteoric water during late highstand system tract formed the unique distribution of halite-moldic pores and vugs. Within the evaporitic epeiric basin, highfrequency sea-level fluctuation and syngenetic exposure are also likely to have occurred, conditions that have been neglected in the classic models. Coupling of the extremely high atmospheric CO2 content, low-amplitude location and high-frequency sea-level fluctuations during the Middle Ordovician greenhouse, together with the flat and broad restricted evaporative environment, controlled the formation of this rare end-member sedimentary assemblage and associated reservoirs. The reservoirs were developed within the sedimentary framework of the North China Platform, which was characterized by a general depression surrounded by three uplifted areas during the Middle Ordovician Majiagou period. The model characterized by epeiric basin-scale disseminated evaporites allowed the formation of reservoirs with high exploration potential. Exploration of this type of reservoir requires that attention is paid to hydrocarbon sweet spots generated by later diagenesis and to heterogeneous hydrocarbon accumulation.

Automated summary

This paper reviews the major genetic environments of evaporite-carbonate assemblages and proposes a new type of reservoir called epeiric basin-scale moldic reservoirs. This rare end-member sedimentary assemblage formed in a unique environment controlled by high atmospheric CO2 content, sea-level fluctuations, and evaporative conditions during the Middle Ordovician greenhouse, providing reservoirs with high exploration potential.