Impact of Tectonic Stresses on Reservoir Quality in a Toe-Thrust Setting of a Tertiary Delta

Joann E. Welton¹, Tina G. Fitts², James M. DeGraff¹, Brian R. Crawford¹, and Ganeswara R. Dasari^1
1 ExxonMobil Upstream Research Company, Houston, TX
² ExxonMobil Exploration Company, Houston, TX

Compaction and cementation are typically assumed to be the primary cause of low porosity at depth. However, in structurally-complex settings, localized elevated mean and shear stresses can accelerate compaction to the point of brittle grain failure resulting in severe porosity degradation prior to cement generation.

To understand the role mechanical processes play in controlling reservoir quality in structurally-complex settings, an integrated reservoir and pressure prediction study was conducted in the toe-thrust region of a Tertiary delta. This study involved combining petrographic data (thin section, SEM & SEM/CL) from core and sidewall samples, with log-based pressure analysis of shales, and physical property analysis to determine the key reservoir quality controls in different structural regimes.

Three main scenarios were recognized: (1) In weak, gently-dipping fold-thrust packages, sands appear to follow a near-uniaxial stress path and have similar reservoir quality to non-thrusted sandstones. (2) In high-angle imbricated thrust packages, mean stresses are elevated, such that sandstones display accelerated compaction (i.e. grain re-arrangement). Brittle grain breakage was observed restricted to narrow bands at actual thrust fault traces. ( 3) In high-angle imbricated, sheared thrust packages, mean stresses are elevated to the point of intense grain re-arrangement and cataclasis (brittle grain breakage). The result is severe porosity degradation due to mechanical failure.