Dolomite genesis and reservoir-cap rock assemblage in carbonate-evaporite paragenesis system

Abstract

Regarding to the problem on the reservoir-cap rock assemblage evaluation in the carbonate-evaporite paragenesis system, this study examined the dolomite and reservoirs genesis and the characteristics of reservoir-cap rock assemblage. Based on the literature research of the global carbonate reservoirs and the case study on four profiles of carbonate-evaporite succession, together with geological and experimental work, three aspects of understandings are achieved. (1) Lithology of carbonate-evaporite paragenesis system is mainly composed of microbial limestone/bioclastic limestone, microbial dolomite, gypsum dolomite and gypsum salt rock deposited sequentially under the climatic conditions from humid to arid, and vice versa, and an abrupt climate change event would lead to the lack of one or more rock types. (2) There developed two kinds of dolomite (precipitation and metasomatism) and three kinds of reservoirs in the carbonate-evaporite system; and the carbon dioxide and organic acid generated during early microorganism degradation and late microbial dolomite pyrolysis process, and early dolomitization are the main factors affecting the development of microbial dolomite reservoirs with good quality. (3) In theory, there are 14 types of reservoir-cap rock assemblages of six categories in the carbonate-evaporite system, but oil and gas discoveries are mainly in four types of reservoir-cap rock assemblages, namely “microbial limestone/bioclastic limestone – microbial dolomite – gypsum dolomite – gypsum salt rock”, “microbial limestone/bioclastic limestone – gypsum salt rock”, “microbial dolomite – gypsum dolomite – gypsum salt rock” and “gypsum dolomite – microbial dolomite – tight carbonate or clastic rock”. These four kinds of reservoir-cap rock assemblages should be related with the climate change rules in the geologic history, and have good exploration prospects.