Origin and anatomy of reservoir cements within a world-class Jurassic source rock

Abstract

The resource potential of organic matter-rich mudstones is not equally distributed throughout a sedimentary basin but varies vertically and laterally as indicated by the differences in gas production, both across regions and even within single wells. Authigenic carbonate (AC) is a microbially-generated, early diagenetic carbonate cement that forms during shallow burial within silty shale pore networks and features prominently as stratabound concretions and laterally continuous hard grounds. AC represents a critical component of every source rock interval because it influences the spatial distribution of permeability while, equally, acting as a primary control on the mechanical properties of the rock (i.e., “fraccability”). While existing research demonstrates that modern ACs account for ~ 10 wt. % of all global carbon burial, new estimates suggest that throughout Earth’s history the volume of AC deposition was significantly higher. In this contribution we present an updated diagenetic framework that explains under which conditions the most significant microbially-driven carbon oxidation reactions occur by employing the Jurassic, shale-dominated Posidonia Shale (Early Toarcian, ~182-174 Ma; ~1-10 wt. % TOC) as a natural laboratory. Significant volumes of AC are produced via a number of reaction pathways including manganese-, iron-, and sulfate-reduction, anaerobic oxidation of methane, methanogenesis, and oxidative decarboxylation, all of which will be considered within our diagenetic framework. The Posidonia Shale (Posidonienschiefer) represents a world-class source-rock that also includes seal and unconventional reservoir facies, which were deposited during an oceanic anoxic event. The Posidonia Shale can be correlated across continental Europe, northern Africa and Canada. Key Posidonia Shale outcrops and drill core from France, Germany and England will feature how regionally changing mud provenance, sedimentation rate and diagenetic pathway determine the boundary conditions for AC formation. Despite being time-equivalent, the Posidonia Shale mudstones regionally develop compositionally a highly diverse assemblage of AC that primarily depend on: (a) pathway of organic carbon oxidation (remineralization of organic carbon versus anaerobic oxidation of methane), (b) availability of inorganic oxidants (manganese, iron, sulfate) (c) brine alkalinity (controlling carbonate nucleation), and (d) the permeability of the diagenetic system (open versus closed system diagenesis).

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019