Abstract
Using observations and descriptions of drilling cores, image logging, microscopic section, argon ion polishing field emission scanning electron microscopy, X-ray diffraction, and whole-rock trace element analysis, the study of shale reservoir diagenesis and space types in the Fengcheng Formation of the Mahu Sag was conducted. Considering the trace element contents and their ratios (Sr/Ba, V/Ni, Th/U, V/(V + Ni), U/Mo, and Sr/Cu), the Fengcheng Formation is formed in a dry and hot continental lacustrine basin with a paleoenvironment of saltwater and anoxic/lean oxygen conditions. The shale reservoirs of the Fengcheng Formation with the characteristic of multisource mixed sedimentation include terrigenous clastic, volcaniclastics, and carbonate rocks. Currently, the Fengcheng reservoir of the Mahu Sag is in the middle of diagenetic-stage B. The principal factors for reservoir densification are compaction, dissolution, carbonate mineral cementation, and clay mineral cementation. The Fengcheng Formation develops multiple reservoir storage space types, such as rock fractures, stylolites, and micro–nanoscale pore-throat systems. The macroreservoir space types include tectonic, induced, bedding, and dissolution–expansion fracture types. The microreservoir space types include microfractures, stylolites, and micro–nanoscale pore throats. The research showed that the Fengcheng Formation has tectonic fracture-pore systems (tectonic fracture-type reservoirs) and stylolite-matrix pore-tectonic microfracture systems (shale oil reservoirs), forming the shale oil preponderant charging channel network. Reservoir space type and its spatial distribution are the principal factors for shale oil accumulation of the Fengcheng Formation in the Mahu Sag.
Highlights
Shale formation is a hydrocarbon source rock and a reservoir that is more compact than sandstone and carbonate tight oil reservoirs (Zou et al 2019a, b). (2) The concept of shale oil is equivalent to tight oil, typically referring to petroleum resources stored in tight oil-bearing formations having low porosity and permeability (Clarkson and Pedemen 2011; Zhang et al 2019a, b, c). (3) Shale oil refers to oil and gas resources in the shale oil and gas system, including organic-rich mudstone and shale as well as the interbedded and adjacent organic-poor reservoirs (Jarvie 2012; EIA 2013; Bowker 2007; Blaizot 2017)
The results showed that the Sr/Ba ratio of the Fengcheng Formation of the Mahu Sag in Fengnan is 0.09–2.95, with an average value of 1.18, and Fengcheng formation is a salty lacustrine basin sedimentary environment
The results showed that the Fengcheng sedimentary period of the Mahu Sag is in the dry and hot continental lacustrine basin, with a saltwater paleoenvironment and anoxic/ lean oxygen conditions
Summary
Great successes in the Bakken shale area in the Williston Basin (Kuhn et al 2012; Pollastro et al 2012; Angster and Sarg 2013), the Eagle Ford shale area in the western Gulf of Mexico Basin (Hentz et al 2014; French et al 2019; Fairbanks et al 2016), the Wolfcamp Formation in the Permian Basin (Ojha et al 2018), the Niobara Formation in the Denver Basin (Han et al 2019), and the Paleogene Green River shale area in the Green River Basin (Burton et al 2014; Sarg et al 2013; Burnham 2017) have changed the oil and gas supply pattern of North America, causingResponsible Editor: Santanu Banerjee1 3 Vol.:(0123456789)2356 Page 2 of 19Arab J Geosci (2021) 14: 2356 et al 2018), and Permian of the Junggar Basin (western China) (Zhang et al 2019a, b, c; Luo et al 2018).Shale oil originally only referred to oil related to oil shale. (1) Shale oil refers to petroleum resources rich in organic shale. Shale formation is a hydrocarbon source rock and a reservoir that is more compact than sandstone and carbonate tight oil reservoirs (Zou et al 2019a, b). (2) The concept of shale oil is equivalent to tight oil, typically referring to petroleum resources stored in tight oil-bearing formations having low porosity and permeability (Clarkson and Pedemen 2011; Zhang et al 2019a, b, c). (3) Shale oil refers to oil and gas resources in the shale oil and gas system, including organic-rich mudstone and shale as well as the interbedded and adjacent organic-poor reservoirs (Jarvie 2012; EIA 2013; Bowker 2007; Blaizot 2017). Shale oil refers to petroleum resources accumulated in the hydrocarbon source-rock strata or shale system. Compaction is critical in the densification of shale reservoirs, and cementation has a certain destructive effect on reservoirs (Petrash et al 2016)
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