From deep basin gas to diagenetic trap—An example from the Yulin Gas Field in the Ordos Basin

Abstract

In the updip portion of the Yulin Gas Field in the Ordos Basin, there are no any structural, stratigraphic and sedimentary lithologic seals. Using thin-section petrography, ultraviolet fluorescence microscopy, micro-thermometry and Raman microspectrometry, this paper finds out the diagenetic trap and studies the diagenetic history of this field. It was revealed that three phases of diagenesis and hydrocarbon charging happened in late Triassic, late Jurassic and the end of early Cretaceous respectively. In the first two phases, acid geofluid entered the reservoir and caused dissolution and cementation. Although the porosities had been increased, further compaction accompanying re-subsidence resulted in tight sandstone and conglomerates. Till the end of the early Cretaceous, bulk of gas migrated into the tight reservoir. Cementation, however, kept on in the updip portion of this field due to low gas saturation and formed the diagenetic trap. The mechanism for gas accumulation was changed from deep basin gas to diagenetic trap, which offers a sealing condition that can retain gas for much longer time.natural gas, deep basin gas, tight reservoir, diagenetic trap, Ordos Basin

Formation mechanism of the upper Paleozoic tight sandstone gas reservoir in the Daniudi gas field, Ordos Basin, China

“…Extensive research has been conducted on the natural gas accumulation of upper Paleozoic gas fields in the Ordos Basin, believing that this region is a continuous or quasi-continuous tight sandstone gas field. The gas reservoir is non-buoyancy driven and exhibits widespread gas generation, near-source accumulation and no obvious gasbearing boundary (Zhang et al, 2000;Dai et al, 2005;Zhang et al, 2007;Zhao et al, 2012;Zou et al, 2013;Lan et al, 2016). With regard to the relationship between reservoir densification and gas accumulation, the general perception is that after tight sandstone reservoirs are formed, natural gas formed in source rocks migrates vertically through fracture systems under excessive pressure and enters reservoirs, gathering near hydrocarbon generation centres (Islam, 2009;Karim et al, 2010;Yang et al, 2012;Xi et al, 2015;Wu et al, 2017).…”