Controls on organic matter preservation and distribution, Upper Cretaceous Colorado Group
Abstract
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.