Doctor of Philosophy
Inconsistencies in production performance in self-sourcing, tight oil reservoirs have been related to lithologic heterogeneities. Applying traditional methods to characterize and model tight-oil petroleum systems presents a challenge as there is an assumption that organic-rich mudstone units are homogeneous with minimal stratal discontinuities. Relating depositional processes that promote organic matter preservation to stratal packaging in a basin evolution context provides the basis for predicting stratigraphic and geographic limits of discrete self-sourcing shale reservoirs. This study uses the regionally extensive organic-rich Blackstone Alloformation, comprising the Fish Scales, Belle Fourche, and Second White Speck formations of the Western Canada Foreland Basin, as a test case to examine the complex relationships between sedimentary facies, basin evolution, and organic matter preservation, with the purpose to determine if organic matter distribution can be predicted by extrapolating sedimentary lithofacies to basin scale.
Correlation of thirteen regionally traceable allostratigraphic surfaces across Alberta and Saskatchewan, covering majority of the Western Interior Seaway, define the Blackstone Alloformation. The resulting framework features tabular- and sheet-like stratigraphic geometries that provide evidence for active tectonic loading in the fold and thrust belt and tectonic quiescence on the stable craton, and the distal basin setting. Minor, isolated thinning and thickening of allomembers indicates that differential subsidence occurred as a response of basement structures to the shifting locations of depocentres. Five primary lithofacies are identified in core and correspond to four lithology categories used for percent lithology mapping: silty argillaceous mudstone, calcareous mudstone, heterolithic sandstone and mudstone, and sandstone. Percent lithology distribution maps show that the Greenhorn marine transgression led to the basinwide deposition of calcareous mudstone and that minor fluctuations in relative sea level from tectonic uplift or eustatic sea level fall meant sedimentation on the low gradient ramp was prone to the reworking of material on the sea floor by storm waves.
Programmed pyrolysis data reveals that the Blackstone Alloformation strata is rich in type II and type II-III kerogen and has high hydrocarbon generation potential. Calcareous mudstone and silty argillaceous mudstone contain the highest wt.% TOC (total organic carbon), followed by heterolithic sandstone and mudstone. Integration of TOC with upscaled sedimentary lithofacies/lithology mapping at the allomember scale confirms that lateral and vertical variations of organic matter preservation and distribution are controlled by factors contributing to the transport and deposition of fine-grained sediments. Enhanced organic matter preservation corresponds to depositional conditions that favour the development of organo-mineralic aggregates and limit organic matter dilution from siliciclastic sediment input or high carbonate production rates.
Summary for Lay Audience
Despite advances in drilling technology that have improved oil and gas extraction from tight, self-sourcing shale-based resources, inconsistencies in production performance have persisted. Recent studies suggest that this variability is related to the heterogeneity of shale and other fine-grained rocks in terms of mineralogy, texture and structure, and its influence on organic matter preservation. The accumulation of organic matter is dependent on the formation of organic-based aggregate particles in the water column and the sedimentary processes responsible for their transport and deposition.
In this thesis, a regionally extensive organic-rich succession of rock spanning Alberta and Saskatchewan is examined to determine the complex relationships existing between sedimentation, basin evolution, and organic matter preservation. The purpose of this work is to determine if organic matter distribution can be predicted by extrapolating distinct physical, chemical, biological attributes of rock units, as observed on vertical scales of centimetres to metres, that show controls on organic matter preservation basinwide. The methodology consisted of creating a time-constrained stratigraphic framework using packages of coeval strata rock packages bounded by key surfaces (discontinuities) and integrating sedimentological and organic geochemistry data into this framework.
Results of this study show that sedimentary processes required to distribute and preserve organic matter are linked to subtle changes in relative sea level. Isopach maps showing thickness variations in rock packages suggest that changes in the thickness of these packages were controlled by the shifting positions of flexural loads in the fold and thrust belt in western Alberta and eastern British Columbia. A rise in relative sea level that accompanied the northward inundation of waters into the Western Interior Seaway from the proto-Gulf of Mexico during the Upper Cretaceous Greenhorn transgression is recorded by the basinwide deposition of calcareous mudstone. Minor fluctuations in relative sea resulting from tectonic movements meant sediment on the sea floor was prone to reworking by storm waves. Integration of organic geochemical data with observations on the sedimentary characteristics of the studied strata revealed that organic matter preservation was enhanced when the environmental conditions in the seaway: 1) promoted development of organic-based aggregates, and 2) limited the input of siliciclastic (silt and sand) and biologically produced (e.g., coccolith and foraminiferal) sedimentary components. These findings contribute to the current understanding of spatial and temporal controls on organic matter distribution and may assist with better predicting the geographic extents of self-sourcing shale-based oil and gas reservoirs.
Flynn, Jessica, "Controls on organic matter preservation and distribution, Upper Cretaceous Colorado Group" (2023). Electronic Thesis and Dissertation Repository. 9338.
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