Abstract

The application of chemostratigraphy to problems in modern and ancient environments has a long and successful history. In particular, the use of high-resolution X-ray fluorescence (XRF) spectrometry for studying the elemental content of core and rock at the sub-millimeter scale to understand provenance, grain size, paleoredox state, terrigenous influence, and other aspects of strata is well documented in paleoclimatology literature. Because it is fast, inexpensive, and non-destructive, XRF analysis has become a popular alternative to more quantitative but slower, more expensive, and destructive sources of elemental data such as Inductively Coupled Plasma methods. Unconventionals were a fast adopter of XRF data, and its use has exploded with the availability of portable devices. In the hydrocarbon industry, XRF analyses are typically used for regional to sub-regional evaluations based on drill cuttings at a coarse sampling interval, typically 90 feet (~27 m). In the past few years, though, companies and researchers have begun to gather finer resolution XRF data.

This paper reports on the interpretation of XRF data measured at a 2-inch (5-cm) interval on conventional core from the Permian Bone Spring and upper Wolfcamp formations in the Delaware Basin in West Texas, a prime target for unconventional hydrocarbon production. Examination of cores reveals that sediment gravity flows from the surrounding margins were the dominant depositional process for moving material into the Basin. Gravity flow event beds were both calciclastic and siliciclastic, and most are near or below wireline log resolution. The heterogeneous nature of the strata, combined with the thinness of many of the beds, creates challenges for log interpretation, log correlation, and prediction of facies and rock properties away from core control.

The high resolution XRF data were acquired to help us build an understanding of the fine-scale cyclicity and patterns in mineralogy, lithology, organic geochemistry, paleoenvironment, and rock properties in order to create a detailed, transferrable understanding of the Bone Spring and upper Wolfcamp formations at sub-log scale. Prior to elemental analysis, it was apparent that interpretation of this dataset would be more involved than those from other unconventional plays such as the Eagle Ford or Haynesville formations, due to the Basin's complex depositional history that delivered huge volumes of shallower water material far into the basin. While several of the products from an unconventional XRF study, such as paleoredox state, come with large uncertainties, the high resolution XRF dataset, combined with complementary data, fills in mineralogic and rock property details between costly routine core analysis data points with a high degree of certainty, facilitating a better understanding of depositional cyclicity, terrigenous influence, grain size, mineralogy, and rock property distribution.

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