Geochemistry of Natural Gases and Condensate in Tazhong Region, Tarim Basin

Abstract

Abstract

A significant petroleum exploration potential has been found in Tazhong region, Tarim Basin, China. This study aims to define genetic mechanisms, migration directions and sources of natural gas stored in Ordovician reservoirs. According to requirements of explorations, whole study region has been divided into three parts, i.e., I, II, and III region.

In I region, natural gas is characterized as typical dry gas and dominated by hydrocarbon gas with high aridity coefficient. The average methane contents are as high as 94%. Methane isotope composition values of natural gas (δ¹³C₁) range between -49.1‰ and -34.8‰ with average value of -39.3‰, indicting highly or over thermal mature. C₂₉20S/(20S+20R)(0.39∼0.63),C₂₉ββ/(αα+ββ)(0.39∼0.78), and Ts/(Ts+Tm)(0.42∼0.92)ratios show that the majority of condensate oil has reached mature stage and several parts are overmature. The difference of maturity between the condensate in I region and typical natural gas suggests a different source or phase. In contrast, the aridity coefficient of natural gas in TazhongIII region is low, with an average methane contents of 84%. The δ¹³C₁ values of natural gas vary in a range of -61.4‰ –-39.1‰ (average ∼ -50.7‰) implying low thermal maturity. In addition, the maturity degree of gas and condensate in II region lies between region I and III. The δ¹³C₁ of natural gas in region II change from -51.6‰ to -31.9‰ with an average of -43.3‰ suggesting a mixture of gases. Due to the impact of mixture, the maturity of natural gas in II region vary in a large range. Partially, the maturity degree of overmature natural gas is overmature significantly different from condensate, showing different sources or phases.

Two sequences of source rocks were deposited in Tarim Basin during Cambrian-Lower Ordovician(highly-over mature)and Middle-Upper Ordovician(mature-highly mature) respectively. Depending on the analyses of maturity gases and condensates in region I are mainly composed of Cambrian-Lower Ordovician oil-cracking gases which migrate from the deep part of Manjiaer depression. The gases and condensates in III region are mainly characterized as the Middle-Upper Ordovician associated gas which come from the mid-deep part of Manjiaer depression. Besides, the gases and condensates in II region originated from a mixture of two source rock sequences. Overall, Different contributions from two sets source rocks led to different characteristics of condensates distributed in Tazhang region.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016