Mapping solid reservoir bitumen by deconvoluting formation mechanism

Abstract

Solid reservoir bitumen is a geologic risk that controls reservoir quality and fluid distribution and has a profound impact on field development and near-field exploration. Understanding the exact mechanism responsible for the development of solid bitumen is therefore a prerequisite toward its proper quantification and mapping. A sandstone reservoir of an anticlinal gas condensate field was investigated geochemically, and data integrated with reservoir diagenetic history to explain abundant solid bitumen encountered within several meters immediately below a regional unconformity in crestal wells, as opposed to deeper sections and downdip wells. The reservoir is generally characterized by a similar sedimentological and diagenetic history across the field, with diagenetic illite grain coats and quartz cement occurring in both updip and downdip wells. The influx of gas condensates into the upper part of the preexisting oil reservoir, possibly facilitated by a major fault, appears to have caused thermodynamic disequilibrium, leading to de-asphalting and subsequent deposition of solid bitumen. Textural and radiometric age dating, integrated with fluid inclusion homogenization temperatures and basin modeling, suggests the initiation of bitumen deposition 129 Ma, immediately after the arrival of gas condensates into the reservoir. As the process was restricted to the uppermost part of the reservoir, it is possible that the solid bitumen, once developed, has improved sealing efficiency along the unconformity and the trapping of subsequent hydrocarbon charges in the underlying reservoir. Better reservoir quality can be inferred farther downdip in areas inaccessible to gas incursions and hence not affected by gas de-asphalting. Other processes, such as regional uplift-induced phase separation, in-reservoir thermal cracking, water-washing, and biodegradation, were also examined and found irrelevant to the formation of solid bitumen in question.