Chapter 3 - Origin and evolution of dolomite reservoirs in the Ordovician Majiagou Formation, Central and Eastern Ordos Basin, NW China

Abstract

The Ordovician Majiagou Formation in the central-eastern part of the Ordos Basin comprises dolomites that form gas reservoirs. These rocks have been investigated by analysis of cores and thin sections, as well as by geochemical analysis. The sediments consist of dolomicrites, very fine-to-fine crystalline dolomite and dolo-arenites; the gas is contained in intercrystalline and dissolution pores. The limestones underwent complex diagenesis with dolomitization, dissolution, filling, and recrystallization. Successive stages of dolomitization can be distinguished. Preservation of primary pores and development of secondary pores made the rock unit an economically exploitable reservoir. Several stages of dissolution strongly influenced the development of secondary pores. Transgression/regression cycles led to the exposure and dissolution of soluble minerals and consequently to secondary pores. The pores were enlarged during burial due to dissolution by organic acids and hydrosulfuric acid from thermochemical sulfate reduction and its product H2S. High H2S concentrations in the study area are therefore considered as possible targets for future exploration.