The Influence of Sedimentation “Rate” and Depositional Processes on Organic-Richness of the Wolfcamp Formation, Midland Basin

Mintz, Jason S. (CGG) | Rich, Barbara (CGG) | Koester, Jens (CGG) | Fenton, Jim (CGG) | Davies, Ceri (CGG) | Harris, Jim (CGG) | Perez, Adriana (CGG)



The process of depositing particulate organic carbon in the rock record is controlled by three main factors which impact organic enrichment through rates of change at the time of deposition of: primary organic production, organic decomposition, and dilution from non-organic (mineral) constituents and biogenic silica or carbonate. Sedimentation rate, which is commonly the least constrained variable, impacts organic matter through: (1) the volume of deposition of non-organic relative to organic constituents and (2) the duration of time that organic matter is exposed to surficial processes whereby organic matter can be lost by oxidation or predation. Depositional processes (tractional vs. turbid vs. debritic vs. hemipelagic) are also controls on the bounding limits of organic-richness of sediments.

Here we examine a core taken through the Sakmarian to Artinskian (Wolfcampian/Leonardian) Wolfcamp B, A (operational stratigraphic divisions) and Dean Formations of the Midland Basin in Reagan County, Texas. This well is in a basinal position ~40km southwest of the paleo-shelf edge at the time of deposition. At the top and base of the studied interval both palynofloral and microfaunal data have been tied to regional events for which absolute ages have been determined and there are two unconformable surfaces for which we have estimated likely durations. Intervals of higher sedimentation correlate with intervals of higher TOC than those with lower sedimentation rates. Other studies in the Midland Basin (and on this core) show relatively low Molybdenum (a proxy for bottom water oxygenation) concentrations which has been interpreted as very low water turn-over rates and restricted conditions in the Midland Basin during Wolfcamp deposition. We hypothesize it is unlikely that organic matter enrichment was promoted by restricted water circulation alone and that high sedimentation rates equate with the rapid burial of organic matter that reduced the duration of exposure to decay at the seabed. Sedimentation is interpreted to have occurred primarily through sediment gravity driven turbidity and hybrid flow processes which limited the degree of hydraulic fractionation of organic matter and thus sediment dilution that could occur. The controls of sedimentation and organic productivity will be examined through climate and source-rock prediction models, a hypothesis for reservoir distribution and an approach towards upscaling these results will also be discussed.