Types and distributional features of Cambrian-Ordovician dolostone reservoirs in Tarim Basin,northwestern China

The Cambrian-Ordovician dolostones are important reservoirs for oil gas accumulations in the Tarim Basin,northwestern China,four types of which are recognized:① tidal flat dolostone or sabkha dolostone,which is dominated by dolomicrite with relatively well developed gypsum-dissolution and interbreccia porosities and was deposited in intertidal to supratidal evaporitic environments.The dolomite show linear correlation of MgO and CaO,low Mg/Ca ratios,high ∑REE with none to dull cathodoluminescene.87Sr/86Sr ratios range from 0.7085～0.7100 and are slightly higher than those of the contemporaneous seawater.The reservoir occurs mainly in Middle-Lower Cambrian and is mainly controlled by sedimentary facies;② evaporitic platform dolomite or seepage-reflux dolostone,which is characterized by reef mound and grainstone fabrics being selectively dolomitized with well developed moldic,gypsum-dissolved and intergranular porosities,shows the wide range of Mg/Ca values,δ13C and δ18O with the values of higher than 2 ‰ and-4 ‰,respectively,and bright red cathodoluminescene.The reservoir occurs mainly in the part of evaporitic platform or lagoon close to platform margin;③ burial dolostone,composed of fine to coarse-crystalline dolomites with well developed inter-crystalline(dissolved) pores,shows relatively negative δ18O with a range from-5‰～-10‰(PDB),higher 87Sr/86Sr ratios ranging from 0.7090 to 0.7110,and dark brown and purple cathodoluminescene.The development of this kind of reservoir was controlled mainly by diagenetic facies,however,tends to be related to the sedimentary facies,because platform margin and inner platform reef and shoal and other open systems with fractures are propitious to the burial dolomitization;④ hydrothermal dolostone,characterized by crystallized dolomite modified by hydrothermal fluid and significantly negative δ18O values less than-9‰(PDB),bright red cathodoluminescene,positive Eu abnormity of REE with homogenization temperatures of fluid inclusions 5 to 20℃ higher than the surrounding strata and irregularly developed enlarged intergranular pores and dissolved vugs and caverns.Hydrothermal dolostone is commonly accompanied by barite,fluorite,pyrite and other hydrothermal minerals.The reservoir occurs mainly along the deep-seated faults under the unconformities.The scale and distribution for the four types of dolostones are different,but they can be predicted using integrated methods.The distribution of burial and hydrothermal dolostones is constrained by both initial depositional facies and diagenetic fluid source.Sabkha dolostone or tidal flat dolostone are variable in scale but can be predicted with the reconstruction of sedimentary environment,diagenetic facies analysis and seismic inversion interpretation.