The likelihood of the organic matter to be preserved with time within rock sediments, is strictly related to the environment in which that organic material was initially deposited. Hence, identifying the paleo depositional environment of an Unconventional reservoir is the first step to estimate the production potential of that play. Once the likelihood of preservation is positively assessed, the actual existence of organic matter needs to be confirmed and characterized in a detail manner.
In this paper, the Authors present a new workflow that allows accurate evaluation of the level of redox conditions of the paleo depositional environment, together with the quantification of the organic matter preserved. Measurements are done quickly at the wellsite, in near-real-time, with limited sampling preparation and whatever the drilling fluids, by analyzing cuttings that are produced during drilling and transported to surface.
The methodology is based on the use of different devices: a powder X-Ray Diffraction (p-XRD) analyzer equipped with a close-coupled transmission based geometry providing quantification of minerals including total clays & micas and total feldspars (Sarrazin, P., and al., 2002), an Energy Dispersive X-Ray Fluoresscence spectrometer, a TOC measurement device and a high resolution digital microscope.
An in-house software has been developed to allow automatic minerals recognition and to accurately estimate their relative proportions. This feature is particularly important for field deployment, where the presence of highly specialized personnel may not be granted. A specific XRD calibration model has been developed, based on a full-pattern approach determined on more than 130 artificial mixtures. A comparison between field and laboratory analysis on field samples confirms the robustness of the mineralogical analysis. An off-shelf Energy Dispersive X-Ray Fluorescence (ED-XRF) spectrometer has been specifically calibrated for sedimentary rocks, to measure majors and mainly traces elements like U, Mo, V, Cu, and Ni. The device allows identifying redox conditions of deposition and flag zones prone to organic matter preservation. In parallel, the organic matter content is estimated in both Oil Based Mud (OBM) and Water Based Mud (WBM), by using a simple combustion and appropriate signal processing to remove the impact of the drilling fluid. No specific sample cleaning is needed besides the basic cutting washing, allowing a very fast screening of the samples. With this workflow, ratios of specific elements provide key indication on water mass restriction and paleo-marine systems.
The integration of these measurements into a robust workflow allows processing 3–4 samples per hour, at the wellsite. This enables taking near-real-time decisions on well placement and/or opportunity of drilling sidetrack, while providing preliminary insights regarding depositional conditions and processes.
Successful applications of this wellsite methodology are presented, and the results of the integration of organic matter content, traces elements and mineralogical composition of the formations are detailed. Several examples from various locations are presented: some from North America and other from other plays around the world.