Diagenesis of a light, tight-oil chert reservoir at the Ediacaran/Cambrian boundary, Sultanate of Oman

Amthor, J. E.; Ramseyer, Karl; Matter, Albert; Pettke, Thomas; Fallick, A. E. (2015). Diagenesis of a light, tight-oil chert reservoir at the Ediacaran/Cambrian boundary, Sultanate of Oman. Geo-Arabia: middle east petroleum geosciences, 20(2), pp. 147-178. Gulf Petrolink

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The Al Shomou Silicilyte Member (Athel Formation) in the South Oman Salt Basin shares
many of the characteristics of a light, tight-oil (LTO) reservoir: it is a prolifi c source rock
mature for light oil, it produces light oil from a very tight matrix and reservoir, and
hydraulic fracking technology is required to produce the oil. What is intriguing about
the Al Shomou Silicilyte, and different from other LTO reservoirs, is its position related
to the Precambrian/Cambrian Boundary (PCB) and the fact that it is a ‘laminated chert‘
rather than a shale. In an integrated diagenetic study we applied microstructural analyses
(SEM, BSE) combined with state-of-the-art stable isotope and trace element analysis of the
silicilyte matrix and fractures. Fluid inclusion microthermometry was applied to record
the salinity and minimum trapping temperatures. The microstructural investigations
reveal a fi ne lamination of the silicilyte matrix with a mean lamina thickness of ca. 20
μm consisting of predominantly organic matter-rich and fi nely crystalline quartz-rich
layers, respectively. Authigenic, micron-sized idiomorphic quartz crystals are the main
matrix components of the silicilyte. Other diagenetic phases are pyrite, apatite, dolomite,
magnesite and barite cements.
Porosity values based on neutron density logs and core plug data indicate porosity in
the silicilyte ranges from less than 2% to almost to 40%. The majority of the pore space
in the silicilyte is related to (primary) inter-crystalline pores, with locally important
oversized secondary pores. Pore casts of the silica matrix show that pores are extremely
irregular in three dimensions, and are generally interconnected by a complex web or
meshwork of fi ne elongate pore throats. Mercury injection capillary data are in line with
the microstructural observations suggesting two populations of pore throats, with an
effective average modal diameter of 0.4 μm. The acquired geochemical data support
the interpretation that the primary source of the silica is the ambient seawater rather
than hydrothermal or biogenic. A maximum temperature of ca. 45°C for the formation
of microcrystalline quartz in the silicilyte is good evidence that the lithifi cation and
crystallization of quartz occurred in the fi rst 5 Ma after deposition.
Several phases of brittle fracturing and mineralization occurred in response to salt tectonics
during burial. The sequences of fracture-fi lling mineral phases (dolomite - layered
chalcedony – quartz – apatite - magnesite I+II - barite – halite) indicates a complex fl uid
evolution after silicilyte lithifi cation. Primary, all-liquid fl uid inclusions in the fracturefi
lling quartz are good evidence of growth beginning at low temperatures, i.e. ≤ 50ºC.
Continuous precipitation during increasing temperature and burial is documented by
primary two-phase fl uid inclusions in quartz cements that show brines at 50°C and fi rst
hydrocarbons at ca. 70°C. The absolute timing of each mineral phase can be constrained
based on U-Pb geochronometry, and basin modelling. Secondary fl uid inclusions in
quartz, magnesite and barite indicate reactivation of the fracture system after peak burial
temperature during the major cooling event, i.e. uplift, between 450 and 310 Ma.
A number of fi rst-order trends in porosity and reservoir-quality distribution are observed
which are strongly related to the diagenetic and fl uid history of the reservoir: the early
in-situ generation of hydrocarbons and overpressure development arrests diagenesis
and preserves matrix porosity. Chemical compaction by pressure dissolution in the
fl ank areas could be a valid hypothesis to explain the porosity variations in the silicilitye
slabs resulting in lower porosity and poorer connectivity on the fl anks of the reservoir.
Most of the hydrocarbon storage and production comes from intervals characterized by
Amthor et al.
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preserved micropores, not hydrocarbon storage in a fracture system. The absence of oil
expulsion results in present-day high oil saturations. The main diagenetic modifi cations
of the silicilyte occurred and were completed relatively early in its history, i.e. before 300
Ma. An instrumental factor for preserving matrix porosity is the diffi culty for a given
slab to evacuate all the fl uids (water and hydrocarbons), or in other words, the very
good sealing capacity of the salt embedding the slab.

Item Type:	Journal Article (Original Article)
Division/Institute:	08 Faculty of Science > Institute of Geological Sciences 08 Faculty of Science > Institute of Geological Sciences > Rock-Water Interaction
UniBE Contributor:	Ramseier, Karl, Ramseier, Karl, Matter, Albert, Pettke, Thomas, Ramseier, Karl
Subjects:	500 Science > 550 Earth sciences & geology
ISSN:	1025-6059
Publisher:	Gulf Petrolink
Language:	English
Submitter:	Thomas Pettke
Date Deposited:	08 May 2015 13:48
Last Modified:	02 Mar 2023 23:26
URI:	https://boris.unibe.ch/id/eprint/68091