Shifting Kerogen Trends in the Midland Basin and what it means for well productivity
Dave Cannon – Diamondback Energy
The late Pennsylvanian – early Permian was marked by climatic shift from wet, tropical environments to drier, arid environments. High frequency cyclicity driven by glacio-eustacy and orogenic forcing in the late Pennsylvanian was gradually replaced by lower frequency cyclicity in the early-mid Permian (Leonardian) when climate patterns shifted in the Permian Basin. It stands to reason that climate change and subsequent depositional environment change would cause variability in organic deposition and preservation. Exhaustive organic petrology can document evidence of kerogen change vertically and laterally within the basin, however it is time and cost prohibitive to develop a truly comprehensive study. Implementing proxies for organic matter classification can increase data coverage and reduce time and cost commitments.
Historically, our industry has utilized routine programmed pyrolysis (RPP) results as a time efficient proxy for organic matter classification. As an initial condition, kerogen has been categorized by macro-depositional conditions (Type I – Lacustrine, Type II – Marine, Type III – Terrigenous). However, subtleties within each major group can occur resulting in a range of organic quality. These are driven by organic depositional mixing in a semi-closed system, organic diagenesis during accumulation and initial preservation, and/or through nutrient level fluctuations that seed initial accumulation; just to name a few. One way to simply track changes in kerogen composition is careful analysis of the hydrogen and oxygen indices obtained through RPP. Unfortunately, most data amassed for this analysis came from samples that were not isolated for kerogen and subsequently extracted. This introduces potential error from mineral matrix effects such as hydrocarbon retention on argillaceous minerals. Our approach takes the oxygen index and subtracts that value from the hydrogen index, which we plot against the thermal maturity proxy, Tmax. The resulting plots identify thermal maturity pathways specific to the kerogen quality group(s) assessed. Even with the errors in measurement in mind, distinct pathways can be interpreted from the clear groups of data.
Over 100 wells with data in the Lower Spraberry and Upper Wolfcamp were assessed in the Midland Basin, whereas over 75 wells were assessed in the Upper Wolfcamp, Upper-Middle-Lower Bone Spring Organic Intervals, and the Avalon Shale in the Delaware Basin. Most of the data originates from protected JIPs, so raw data will not be presented unless it was acquired solely by Diamondback. There appears to be distinct differences in our interpretive plots between the Midland and Delaware Basins. There is a much wider range in values across the Midland Basin, from low initial hydrogen to high initial hydrogen. Interpreted in this study are four distinct maturity pathways in both the Upper Wolfcamp and Lower Spraberry. However, in the Delaware Basin this range narrows and illustrates a general decrease in initial hydrogen indices for all kerogen assessed in the Wolfcamp and Bone Spring.
Initially, one would attribute this to different kerogen “types”. However, typing kerogen into lacustrine, marine, terrigenous, and residual is a slippery slope that can introduce a large amount of bias into a geochemical assessment. Most reference kerogen utilized to construct the often used (and abused) modified Van Krevelen diagram come from a narrow band of samples and basins, and does not truly depict the wide variety in organic depositional and preservation environments. One source of confusion is the fact that typing infers a chemical group, not a depositional or optical classification group. The fact that Type II is classified as marine, liptinitic, or even algal is narrow sighted and ignores the complexity of different environments. Tissot (1984) observed these problems, “Systematic elemental analysis performed on a set of amorphous kerogens from various origins has shown that, although some of them do belong to “type II”, the chemical composition of the amorphous kerogen may spread over the entire Van Krevelen diagram.” With this ambiguity in mind, how can we properly link the chemical analysis to depositional analysis?
By assessing the results of our RPP interpretations and combining them with produced product assessment and detailed stratigraphic analysis of both basins, we can reasonably demonstrate depositional forcing mechanisms influencing the diversity in kerogen quality/chemical group. Furthermore, we can link produced product analysis to the various kerogen quality/chemical groups across Midland Basin wells. Work in the Delaware is ongoing. In the Midland Basin, we observe distinct relationships with depressed hydrogen indices and increased oxygen indices in areas of sediment transport. If we assume minimal migration of hydrocarbon in these coupled mudrock/gravity flow bodies, there is a further relationship between light oils and kerogen quality/chemical group. Alternatively, in areas of lower sediment transport (highly mudrock dominated), higher hydrogen indices and heavier oils tend to be common. Interestingly, the thermal stress required between these two major groups are different. To create the same gravity and quality of fluid, lower thermal stress is needed for our “mixed” group as compared to our “pelagic” group.
This workflow has exploration and exploitation ramifications. By being able to elicit differential fluid properties by kerogen quality/chemical group, dependent on thermal stress required, high-grading of acreage positions or unveiling of previously overlooked organic mudrocks can be accomplished. Noting that the “mixed” kerogen group requires less thermal stress to produce lighter products, the pre-conceived edges of a prospective basin can be pushed further out to accommodate the potential productivity of these rocks. Conversely, this workflow can condemn portions of the basin that appear, at first, to have sufficient maturity. On closer inspection, these areas may not produce the quality of product necessary to flow at commercial rates through low permeability reservoirs. In other words, throw out all your maturity maps. Sorry.