Dolomite porosity related to peritidal microbialites textures and facies succession in Mesoproterozoic Wumishan Formation, North China

Abstract

There are three main types of microbial dolostones in Mesoproterozoic Wumishan Formation in Jixian, North China. They are stromatolite, thrombolite and granular microbialite. The microbialite lithofacies are intensely environmentally associated. Several kinds of stromatolite can be found in the peritidal environments. Mostly planar or wavy stromatolites show in intertidal and supratidal zones while domy and conic stromatolites occur in subtidal or lower intertidal zones. In more agitated subtidal zone or tidal channels, microbial grains deposited, such as thrombolite, oncolite, and sand- gravel sized microbial-coated grains. The intraclasts related laminite facies mostly deposited in shallow subtidal zone.Representative four outcrop sections of Wumishan Formation in Jixian are measured and documented. The petrophysical and petrological study from the outcrop and well core samples reveals that the reservoir rocks are closely related to lithofacies and their depositional environments. The more agitated the depositional environment is, the better the reservoir quality, even so that after the diagenesis (surface and burial). The dolostone porosity of planar stromatolite in supratidal and intertidal are much worse than the subtidal high-agitated microbialite rocks, whether samples from the outcrops or drilling cores.The effective reservoir interval is normally located in the lower part of a peritidal microbial facies succession, mainly consists of subtidal microbialite, such as thrombolite, thrombo-stromatolite, oncolite, columnar stromatolite and coated intraclastic grainstone and rudstone, commonly deposited in agitated environment. The thickness of reservoir rocks in microbialite para-sequences ranges commonly from 4 to 14 m, deeply related to depositional cycles or sequence stratigraphy in vertical lithofacies texture and their areal extent to the paleogeography.