Lithological and diagenetic restrictions on biogenic gas generation in Songliao Basin inferred from grain size distribution and permeability measurement

Abstract

Organically rich immature source rocks occur commonly at shallow burial depths in Songliao Basin, NE China. This has led to the general assumption that there must be a large biogenic gas resource in these organic rich, shallow strata. However, except for a few small gas fields that are generally associated with biodegraded oil, only a small amount of biogenic gas has been discovered. In order to determine the geological factors controlling biogenic gas generation and accumulation, a suite of potential source rock shale samples from burial depth less than 1600 m was analyzed for organic composition, grain size distribution and permeability. The results show that selected samples are organic rich and thermally immature. The grain size analysis shows a mixed distribution of silt and clay with D10 generally below 1.6 µm. The measured permeability values are generally below 1 µd under in situ stress conditions with pore throats that are most likely smaller than that of methanogenic bacteria. Very low permeability and very small pores resulting from compaction and diagenesis restrict bacteria movement and activity, limit nutrient transport, diminish space availability, and lead to a reduced biodiversity. Currently, no microbes can survive in these Cretaceous shales even though the shales never reached geopasturization temperatures and there is sufficient substrate. Biogenic gas generation is inferred to be restricted to near surface strata while elsewhere in this succession methanogenesis has ceased, which significantly reduces the exploration potential for biogenic gas. RésuméLes roches mères immatures riches en matières organiques se retrouvent couramment à faible profondeur dans le bassin de Songliao dans le Nord-Est de la Chine. De ce fait, on présume généralement qu'il doit exister d''importantes ressources de gaz biogéniques dans ces strates peu profondes abondantes en matières organiques. Cependant, à l'exception de quelques champs gazéifères de faible envergure liés généralement à du pétrole en biodégradation, seule une modeste quantité de gaz biogénique y a été découverte. Afin de déterminer les facteurs géologiques qui régissent la production et l'accumulation de gaz biogénique, des échantillons de roches mères potentielles enfouies à moins de 1600 m ont été analysés en vue d'en connaître la composition organique, la granulométrie et la perméabilité. Les résultats montrent que les échantillons de la séquence de shale se révèlent riches en matières organiques et immatures sur le plan thermique. La granulométrie expose un mélange de silt et d'argile avec le D10 affichant une taille inférieure à 1,6µm. Les valeurs de la perméabilité mesurée sont généralement inférieures à 1µd dans des conditions de contraintes in situ avec des pores vraisemblablement plus petits que ceux des bactéries méthanogènes. La perméabilité très faible et de très petits pores résultant de la compaction et de la diagenèse restreignent le mouvement et l'activité des bactéries, limitent le transport des nutriants et diminuent l'espace, ce qu...

Acetate, DIC and δ13CDIC evidence for acetoclastic methanogenesis in Songliao Basin, NE China

“…Majority of biogenic gas (δ 13 C CH4 o À 55‰) can be attributed to oil biodegradation but gases from dispersed immature source rocks are projected . Data from present study are consistent with previous publications Li et al, 2015) that primary biogenic gas is limited.…”

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