The influence of multiple-stage oil emplacement on deeply buried marine sandstone diagenesis: A case study on the Devonian Donghe sandstones, Tabei Uplift, Tarim Basin, NW China

Abstract

Abstract Understanding the oil emplacement influence on diagenesis is one of the most key factors of reservoir quality prediction. The Devonian Donghe sandstones are deeply buried to 5700～6000 m, and they are important exploration target in the Tarim Basin, additional the reservoirs experienced multi-phase hydrocarbon accumulation and destruction. The Donghe sandstones contain various diagenetic minerals, which are mainly composed of calcite cement, a small amount of quartz overgrowth and pyrite, trace ankerite and ferrocalctie. The current study investigates the influence of multiple-stage oil emplacement on the diagenesis with particular attention to calcite and quartz cementation of marine clastic reservoir interval of the Donghe sandstones in the Tarim Basin. The samples in the oil leg and water leg have been collected and a series of studies were performed, including thin section petrography, scanning electron microscopy (SEM), carbon and oxygen isotopes and cathodoluminescence (CL) analysis. The δ13CV-PDB values of calcite cement from the oil leg and water leg between −4.73 and −1.21‰ and −3.81 to −0.86‰, respectively. The δ18OV-PDB values of calcite cement from the oil leg and water leg range from −15.59 to −6.61‰ and −14.97 to −7.76‰, respectively. Both of them display no obviously different in the oil leg and water leg. All of the δ13CV-PDB values in the whole interval of the Donghe sandstones show lightly depletion was explained mixing from marine carbon and carbon produced from bacterial sulfate reduction (BSR) that happens during petroleum degradation associated with sulfate reduced by hydrocarbons bacterially. Two dull-luminescence zones, trace Fe and Mn content, and high Fe accompanied by low Mn content, and orange-red- and bright-orange-yellow-luminescence zones in oil leg indicate four generations of calcite occurred, whereas two generations of calcite occurs in water leg, which are dull- and orange-red-luminescence zones. Two origins of calcite cement were clearly identified: marine biogenic carbonates and hydrocarbon-related from palaeo-reservoir BSR during uplift. The first generation of calcite cement originated from biogenic carbonate, the second and third generations of calcite cement were mixing from predate carbonate cement dissolved and re-precipitated, and calcite precipitated by BSR directly. Some quartz grains and K-feldspars were dissolved by residual organic acids that remained in the irreducible water, and following replaced by the fourth generation of calcite cement. Some calcite cement in the oil leg precipitated in a temperature higher than the homogeneous temperature of fluid inclusion, which indicates oil emplacement has no influence on calcite cementation. The quartz overgrowth postdate the first hydrocarbon accumulation and predate the second hydrocarbon accumulation, which originated from quartz grains dissolved by organic acids that produced from BSR and re-precipitated at a low temperature ( The current study can promote the understanding of the influence of multiple-stage hydrocarbon accumulation on diagenesis, and it can also inspire the diagenesis process of other Paleozoic hydrocarbon reservoirs in the Tarim Basin.