Geochemical characteristics and its paleo-environmental significance of the Lower Cambrian carbonate in the northwestern Tarim Basin: A case study of Well Shutan-1

Abstract

Recently, subsalt exploration in the Tarim Basin has been one of the most significant targets of the deep hydrocarbon exploration.The Lower Cambrian Xiaoerbulake Formation is the major reservoir in current exploration.However, the analysis of the detailed geochemical and paleoenvironmental characteristics are limited in Keping area in absence of studies in the interior of the basin.This paper focuses on the Well Shutan-1 in the Bachu Uplift of the northwestern Tarim Basin.Based on the analysis of petrology and the geochemical characteristics (major, trace, rare elements and carbon-oxygen isotope), depositional environment of the Lower Cambrian Xiaoerbulake Formation is explored.Our study indicates that the lower, middle and upper part of the Xiaoerbulake Formation in Well Shutan-1 comprise thick algal dolomite, mudstone interbedded with dolomitic limestone and fine to silty dolomite, respectively.The average value of Lan/Ybn is 1.09 in the Xiaoerbulake Formation, of which the lower and upper part show positive Eu anomaly.Silica cementation, silica fragment and dissolution can be seen in the thin section, indicative of hydrothermal influence.Redox-sensitive trace elements are enriched in the middle of the formation.V/Cr and Ni/Co range from 2 to 4.25 and 5 to 7.The grain size of the sediment is finer.These are indications of suboxic bottom-water condition.The RSTEs and their ratios are relatively low in the upper and lower Xiaoerbulake Formation.The grain size of sediment is coarser.These are indications of oxic bottom-water condition.The carbon isotope ranges from -1.3‰ to 2.7‰. It decreases firstly and then increase throughout the interval.Two positive and one negative isotope excursion are identified in the lower and middle part of the interval, respectively.Paleoenvironmental reconstructions indicate that paleo-salinity firstly increased and then decreased.Paleo-seawater temperature gradually increased.Dolomite reservoirs were well developed in the late stage due to moderate salinity, occurrence of algal dolomite along with later weathering and dissolution.The research results can provide a basis for oil and gas exploration in this area.

Keywords

• early cambrian
• xiaoerbulake formation
• geochemistry
• carbon isotope
• paleoenvironment