Porosity distribution in cyclic dolomites of the Lower Qiulitag Group(Upper Cambrian)in northwestern Tarim Basin,China

Increasing interests in hydrocarbon resources at depths have drawn greater attentions to the deeply-buried carbonate reservoirs in the Tarim Basin in China.In this study,the cyclic dolomite rocks of Upper Cambrian Lower Qiulitag Group from four outcrop sections in northwestern Tarim Basin were selected to investigate and evaluate the petrophysical properties in relation to depositional facies and cyclicity.The Lower Qiulitag Group includes ten lithofacies,which were deposited in intermediate to shallow subtidal,restricted shallow subtidal,intertidal,and supratidal environments on a carbonate ramp system.These lithofacies are vertically stacked into repeated shallowing-upward,meter-scale cycles which are further grouped into six third-order depositional sequences(Sq1 to Sq6).There are variable types of pore spaces in the Lower Qiulitag Group dolomite rocks,including interparticle,intraparticle,and fenestral pores of primary origin,inter crystal,and vuggy pores of late diagenetic modification.The porosity in the dolomites is generally facies-selective as that the microbially-originated thrombolites and stromatolites generally yield a relatively high porosity.In contrast,the high-energy ooidal grainstones generally have very low porosity.In this case,the microbialite-based peritidal cycles and peritidal cycle-dominated highstand(or regressive)successions have relatively high volumes of pore spaces,although highly fluctuating(or vertical inhomogeneous).Accordingly,the grainstone-based subtidal cycles and subtidal cycle-dominated transgressive successions generally yield extremely low porosity.This scenario indicates that porosity development and preservation in the thick dolomite successions are primarily controlled by depositional facies which were influenced by sea-level fluctuations of different orders and later diagenetic overprinting.