The Identification and Origin of Diamondoid and Thiadiamondoid Products From Condensate of LS2 Well, Tarim Basin: Significance for Oil Exploration Potential of Ultra-Deep Strata

Abstract

High gas production was discovered last year (i.e., 2016) from the LS2 well in Bachu uplift of the Tarim Basin, China identifying a promising new region for petroleum exploration. The very S-rich petroleum condensate contained high concentrations of H₂S (3.66%) and distinctive organic sulfur compounds useful for future oil correlations. Here we report the detection of vast distributions of thiophenic, sulfidic and diamondoid products which were resolved by comprehensive two-dimensional gas chromatography-time-of-flight mass spectrometry (GC²-TOFMS), including a much larger number of alkylated diamondoid and thiadiamondoid isomers than previously reported from petroleum analyses. These caged molecular structures are typical secondary petroleum products of thermochemical sulphate reduction (TSR). A d³⁴S distinction between OSCs reflecting a varied sensitivity to TSR (i.e., d³⁴S_BT-DBT ~ 4 ‰) and hydrocarbon gases reflecting a dry composition (C₁/C₁₊ ~ 1) and ¹³C enrichment with δ¹³C values (e.g., C₁ ~ -34 ‰ and C₂ ~ -30 ‰) up to 8 ‰ heavier than non TSR impacted Tazhong condensates - represent further evidence of TSR impacts on the LS2 condensate. The LS2 well lacks SO₄^2- evaporites to directly have supported TSR, implying it is a secondary reservoir of migrated gas. Furthermore the reservoir temperature (144 °C) is relatively low for TSR. The d³⁴S values of the oil and gas are typical of deeper Cambrian strata which contain high quality Xiaoerblak (∈_1x) dolomites, thick evaporites and experienced temperatures greater than 200 °C from which TSR derived petroleum could have migrated through fracture adjustment of the late Himalayan period. This discovery suggests the Cambrian salt layers may be a productive source of condensate rich petroleum warranting further exploration.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018