The effect of slight to moderate biodegradation on the shale soluble organic matter composition of the upper triassic Yanchang formation, Ordos Basin, China

Abstract

Biodegradation greatly affects the physical properties and economic value of oil and gas reserves. Here, the composition of a suite of 27 samples of shale soluble organic matter from typical biodegraded shale profiles in the Upper Triassic Yanchang Formation, Ordos Basin, China, was analyzed using GC/MS/MS and ESI FT-ICR MS technology. The effects of slight to moderate biodegradation on the compositions of saturated hydrocarbons and polar nitrogen, sulfur, oxygen (NSO) compounds were assessed, revealing three key effects as biodegradation increases (1) Chloroform asphalt "A" (i.e. soluble organic matter). and saturated hydrocarbons gradually decrease, aromatic hydrocarbons gradually increase, non-hydrocarbons increase, and there is no regular change in asphaltene content. (2) The relative content of pristane (Pr) and phytane (Ph) to their corresponding n-alkanes increases gradually, indicating the n-alkanes decrease gradually. However, the resistance of the n-alkanes to biodegradation may be related to the carbon number and odd-even distribution. As the carbon number increases, the ability of n-alkanes to resist biodegradation increases, and even carbon number n-alkanes may be more resistant to biodegradation than odd carbon number n-alkanes. (3) No obvious changes in the relevant biodegradation indices of steranes and polar NSO compounds were observed in the entire profile. Hopane biodegradation indices were variable and sometimes contradictory with depth throughout the sediment profile. However, overall the data suggest the hopanes were not affected by biodegradation.Our results indicate that shale oil can also be biodegraded, and due to the tightness of shale reservoirs, slight to moderate biodegradation is common. Changes in shale oil content in the profile are the result of the primary migration and biodegradation together, with primary migration occurring mainly from the shale to the upper sandstone reservoirs. The above evidence shows that slight to moderate biodegradation has a relatively small impact on shale oil. However, in previous work we have shown that biodegradation has significant impacts on the shale gas content and its geochemical characteristics.