Genesis and role of bitumen in fracture development during early catagenesis

Abstract

Bitumen-bearing fractures and vugs were investigated in the highly organic-rich Jordan Oil Shale (JOS) of Late Cretaceous to Eocene age, which has potential as a highly fractured, unconventional hydrocarbon play. Bitumen is present as macroscopically visible deposits and as inclusions in the cement of abundant natural fractures and adjacent vugs. The frequency of bitumen occurrence in fractures closely correlates with Total Organic Carbon (TOC) and burial depth. Petrographic and organicgeochemical analyses on bitumen samples extracted from fractures and their host rock matrix show that the fracture-filling bitumen comprises indigenous low maturity hydrocarbons derived from the surrounding organic-rich Oil Shale and has not migrated from a deeper source. Maturity indicators imply that the oil shale is in the pre-oil generation stage of early catagenesis throughout the investigated area, but with a regional increase in thermal maturity from west to east as the result of greater maximum burial depth. Bitumen mobilization in the host rock was mainly controlled by vertical loading stress acting on the non-Newtonian bitumen phase in load bearing configurations in the organic-rich matrix.Bitumen fractures were developed by hydraulic fracturing as the result of fluid overpressure in the organic matter. Overpressured bitumen has acted as a fracture driver, generating bitumen veins in both the organic-rich mudstones and the adjacent chert and silicified intervals.

Hyrous Pyrolysis of Source Rock Plugs: Geochemical and Visual Investigations and Implications for Primary Mirgation

“…If the fracture criteria are reached, micro-fracturing of the rock occurs, thereby providing new migration pathways. Such microfractures have been frequently observed in petroleum source rocks of different maturity levels (Littke et al, 1988;Abu-Mahfouz et al, 2019, 2020. The conversion of solid kerogen towards liquid hydrocarbons itself creates new void space, which can facilitate pressure-driven migration.…”

Stress distribution around kerogen particles as a measure of the initiation of bitumen-filled microfractures in organic-rich source rocks

“…For a normal stress regime dominated in Oligocene to Miocene during bitumen generation [ 1 , 2 ], the maximum principal stress is the overburden load while the horizontal stress is the least. The effective principal stresses acting on the kerogen boundary can be approximated as follows: where is the far-field overburden/vertical stress, is the hydrostatic pore pressure, and is the biot-Willis coefficient [6] .…”

“…Table 1 presents the physical properties for the lower interval used in the calculation. These are obtained from 1) direct laboratory measurements, 2)testing by Taqieddin & Al-Homoud [9] , and 3) from other previous studies (e.g., [ [2] , [3] , [4] , [5] , 7 , 8 , 10 ]). A hypothetical estimate of Young's modulus property adapted from other known carbonate mudstone source rocks is applied to the calculation, with uncertainty values ranging from 16 to 40 GPa.…”

“… Properties Units Values Sources Kerogen density (kg/m3) 1100 Forsman & Hunt [5] Bitumen density (kg/m3) 1010 McCain [8] Rock density (kg/m3) 1715 Taqieddin & Al-Homoud [9] Water compressibility (MPa-1) 0.0004 Berg & Gangi [3] Kerogen compressibility (MPa-1) 0.0015 Dubow [4] Bitumen compressibility (MPa-1) 0.0022 McCain [8] Initial total organic carbon 0.3 Abu-Mahfouz et al. [2] TOC to organic matter transformation coefficient ∼1.3 1.3 Vernic [10] Porosity 0.1 Taqieddin & Al-Homoud [9] Volumetric content of original kerogen 0.4575 Calculated, Vernic [10] <...…”

Modelling the initiation of bitumen-filled microfractures in immature, organic-rich carbonate mudrocks: The Maastrichtian source rocks of Jordan

“…Previous studies on these source rocks have focused on their organic matter enrichment, maturation, hydrocarbon potential, sedimentological, and paleoenvironmental implications (e.g., Abed and Al-Arouri., 1994;Abed et al, 2009;Ali Hussein et al, 2014a;März et al, 2016;Hakimi et al, 2016Hakimi et al, , 2018. Only very few studies have focused on the bitumen disseminated in the matrix and filling hosted fractures (Abu-Mahfouz et al, 2019, 2020. Abu-Mahfouz et al (2019) conducted a comprehensive geochemical investigation of bitumen fractures in these Upper Cretaceous Source Rocks and presented a conceptual model favouring the development of bitumen fractures by hydraulic fracturing.…”