The University of Texas at Austin
Over the last two decades studies have emphasized the underappreciated impact of bottom currents and point-sourced gravity-driven currents in deepwater allochthonous carbonate depositional environments, leading to re-interpretations of many previously examined systems. Wolfcampian and Leonardian intervals in the Permian Basin provide a thick succession of basinal strata to re-evaluate with updated insight and models. Subsurface mapping and cross sections highlight multiple mounded and elongate carbonate-mud rich accumulations reaching up to 800m thick along the western slopes of the Delaware Basin. The mounded nature, lack of platform-focused sourcing, and presence of oblique cross-cutting channels suggest these features represent carbonate contourite drifts generated by bottom currents as opposed to gravity-driven deposits. Thickness relationships also indicate bottom currents may locally sweep sediments from toe-of-slope environments, inhibiting platform progradation.
In the deeper basin, mapping highlights previously undocumented point-sourced carbonate accumulations interpreted as calciclastic submarine fans. Some fans extend into the basin over 100km and reach 250m in thickness. The fans range in composition, containing both carbonate debrites and turbidites with varying volumes of mud-rich siliciclastic deposits. Runout of gravity flows resulted in concentration of comparatively coarser carbonate material in proximal fan environments and finer siliciclastic mudstone, siltstone, and organic matter in fan fringe environments.
The findings are significant as the processes responsible for drift and fan formation impart significant control on facies distribution and subsequent reservoir quality, reservoir architecture, organic matter distribution, and vertical and lateral distribution of barriers to induced fracture growth. Correct characterization across multiple zones may highlight stacking of sub-environments to explain production trends, identify advantageous locations for multi-zone development, or inform spatial differences in horizontal well spacing and stacking. Lastly, as these new interpretations recognize calciclastic drifts and fans in one of the most heavily studied and geologic data-rich areas in the world, it indicates these systems are likely overlooked and more common in the ancient rock record.