--> Microfacies Relationships of Organic-Rich Mudstones: Implications for Depositional Processes and Paleoenvironment Interpretations of Mud-Dominated Successions in Ancient Epicontinental Seas

AAPG ACE 2018

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Microfacies Relationships of Organic-Rich Mudstones: Implications for Depositional Processes and Paleoenvironment Interpretations of Mud-Dominated Successions in Ancient Epicontinental Seas

Abstract

Laboratory experiments and sedimentological studies of ancient and modern mudstone successions over the past decade have shown that mud can form aggregates and be transported as bedload at current velocities required to transport and deposit sand. This challenges the traditional view that mudstones are homogenous suspension settling deposits and instead proves that fine-grained successions are deposited by a wide variety of depositional processes and exhibit the same degree of heterogeneity as their coarser-grained counterparts. Recognizing these processes has important implications for characterizing the heterogeneity of mudstone successions ad the vertical and lateral distribution of mudstone facies will have strong controls on reservoir heterogeneity.

The organic-rich succession of the Second White Specks Formation is the interval of interest as it is a major source rock and unconventional play that spans 100s of square kilometres across the Western Canadian Sedimentary Basin. The core used for this study is of exceptional quality and covers over 100m of fine-grained strata. Most mudstone sedimentology studies are limited to weathered outcrop or old core where thin sections are required to discern subtle sedimentary structures within the fine-grained intervals. Although thin section analysis is essential for describing the small scale sedimentary structures and compositional changes, they are still point samples that do not fully characterize the relationship between microfacies over the entire length of the study interval. The dataset presented in this study provides a rare opportunity to characterize a mudstone succession in detail because of the pristine nature of the core described. Detailed descriptions of grain size, mineralogy, and sedimentary structures were conducted at the mm to cm scale for a 100m interval from core and thin sections.

Results from this study support the hypothesis that the interval of interest was deposited in relatively shallow-water (above storm wave base) under dynamic conditions, rather than in a quiescent, relatively deep water environment as suggested by traditional models for mudstone deposition and biostratigraphy. Similar sedimentary structures are observed throughout the succession despite there being significant compositional changes. This indicates that proximity to sediment sources has a greater control on the accumulation and preservation of organic-matter than the processes that deposited the sediment.