Towards an Unified Mudstone Model: Common Attributes and Controls on Hydrocarbon Source, Reservoir, and Seal Potential

Bohacs, Kevin M.¹ (1) ExxonMobil Upstream Research Company, Houston, TX

Most of the attributes that make a good source rock also make good reservoirs and seals. Economically effective source, reservoir, and seal mudstones all have high clay content, parallel-laminated fabrics, significant pelagic organic-matter content, and early diagenetic cements. In addition, good seals generally have silt content < 20% and ductile components, whereas shale-gas reservoirs are dominated by planktonic input that yields brittle lithofacies. A significant thickness of each facies and advanced state of thermal maturity is required for economic effectiveness. These shared attributes allow us to leverage insights and models developed for source rocks to predict seal and reservoir facies.

Biogenic input is controlled by insolation, nutrients, and water supply. Predictions rely on reconstructions of paleogeographic, paleoceanographic, and paleoclimatic conditions (e.g., water depth, paleolatitude, upwelling, rainfall, runoff). Organic matter accumulation is another key shared parameter and predictions need to reconstruct sea-floor energy and ecosystem conditions, and rates of oxidant supply and burial, along with biogenic input. Early diagenesis is similarly influenced by rates of organic matter input and burial, and sea-floor and upper sediment column conditions, along with supply of terrigenous weathering products.

Shared controls also highlight key unanswered questions about processes affecting mudstone accumulation. It forces closer examination of what are primary causes as opposed to secondary consequences, i.e., is sediment-column anoxia a cause or consequence of organic-matter accumulation? Fundamental work on processes is needed, especially on exactly how mud behaves during transport and on how organic matter accumulates and how it affects diagenetic conditions during early burial.