| APPLICATION OF INORGANIC WHOLE ROCK GEOCHEMISTRY TO SHALE RESOURCE PLAYS: AN EXAMPLE FROM THE EAGLE FORD SHALE FORMATION, TEXAS. |
| Unconventionals |
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1Chemostrat Ltd, 2Pioneer Natural Resources
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| Introduction |
Over the past decade, shale resource plays have risen to the forefront of hydrocarbon exploration. However, the fine grained, macro-scale homogeneity of many shale plays currently being exploited has negated some of the more traditional approaches to reservoir characterization and stratigraphic correlation, resulting in the search for new methodologies that enable better understanding of shale reservoirs. Here, the application and potential of one approach is demonstrated, namely the application of inorganic whole rock geochemical data to shale resource plays. Inorganic whole rock data from the Eagle Ford Formation are used to create a chemostratigraphic correlation framework, model bulk mineralogy, model TOC data, better understand paleoredox conditions and to provide information on relative rock brittleness, all aspects that are key to understanding shale reservoirs.
Inorganic whole rock geochemical data have been used to define stratigraphic correlations in the petroleum industry for over a decade now (Ratcliffe et al., 2010 and references cited therein) The stratigraphic technique of chemostratigraphy relies upon recognizing changes in element concentrations through time and using those to model changes in geological features, such as paleoclimate (Pearce et al., 2005, Ratcliffe et al., 2010) and provenance (Ratcliffe et al, 2007, Wright et al,. 2010). Published accounts using this approach are largely on fluvial successions, where stratigraphic correlation using traditional techniques are often problematic (e.g. Pearce et al., 2005, Ratcliffe et al., 2006, Ratcliffe et al., 2010, Wright et al., 2010 and Hildred et al., in press). Over the same decade, inorganic whole rock geochemical datasets have routinely been acquired from organic-rich mudrocks, the data typically being used to help elucidate paleoredox conditions during oceanic anoxic events (e.g. Tribovillard et al., 2006, Turgen and Brumsack 2006, Tribovillard et al,. 2008, Negri et al., 2009, Jenkyns, 2010). Here, approaches of the chemostratigraphic workers and the oceanic anoxic event workers are combined and pragmatically applied to the Eagle Ford Formation in Texas, USA. |
| METHODOLOGY AND DATASET |
The Eagle Ford Formation is a dark grey, calcareous, locally organic-rich mudstone of Cenomanian – Turonian age that is sandwiched between the Cenomanian-age Buda Formation and the Coniacian-Santonian-age Austin Chalk Formation. The study area, in south Texas, forms a narrow strip that extends from La Salle County in the SW to Lavaca County in the NE, a distance of >150 miles. Over this distance, the Eagle Ford Shale Formation varies in thickness from approximately 75ft to 300ft. For this paper, over 500 samples from 11 wells have been analysed using inductively coupled plasma optical emission (ICP-OES) and mass spectrometry (ICP-MS), following a Li-metaborate fusion procedure (Jarvis and Jarvis, 1995). These preparation and analytical methods provide data for 10 major elements, 25 trace elements and 14 rare earth elements. Precision error for the major element data is generally better than 2%, and is around 3% for the high abundance trace element data derived by ICP-OES (Ba, Cr, Sc, Sr, Zn and Zr). The remaining trace elements are determined from the ICP-MS and data are generally less precise, with precision error in the order of 5%. |
Stratigraphic characterization and correlation Developing stratigraphic frameworks is the key to the exploration for and exploitation of any hydrocarbon basin. In shale plays, the more traditional methods to stratigraphic correlations used by the petroleum industry are often limited. Commonly, the restricted basin nature of their accumulation can limit the use of biostratigraphy and palynomorphs are often thermally degraded. Electric log correlations are hampered by high, but erratic U values that reflect a mixture of detrital input and authigenic enrichment from sea water. Furthermore, the apparent macro-scale homogeneity of the mudrocks precludes the recognition of sedimentary facies that can be used for stratigraphic correlations, particularly when the only samples available are cuttings. Figure 1 displays the chemostratigraphic characterization of the Eagle Ford Formation in well Friedrichs #1 and Figure 2 the extension of that characterization into the 5 wells of the 11 in the study.
Once a robust chemostratigraphic correlation is achieved, it can also be used as a basis for determining the well pathways in horizontal multilateral wells, either post-drill or at well-site (Schmidt et al. 2010).
 Figure 1. Chemical logs constructed for elements and element ratios used to define chemostratigraphic packages and geochemical units. Each square represents the location of an analysed sample.. An important aspect to understanding shale reservoirs is determining their mineralogy and their TOC contents. Typically, this is achieved using X-Ray diffraction and LECO analysis respectively. However, major element geochemistry can been used to provide semi quantitative mineralogical data (Paktunc 2110, Rosen et al., 2004). Here, bulk mineralogy calculated from whole rock geochemical data are compared against mineralogical data acquired from XRD to demonstrate the strengths and weaknesses of using calculated mineralogy. Similarly, semi quantitative TOC values can be calculated from trace element geochemistry by calculating a linear regression equation between selected trace elements and measured TOC. Provided the relationship between trace elements and TOC has a regression coefficient of over 0.8, it can be used to model TOC values where LECO determinations have not been made.
 Figure 2. Chemostratigraphic correlation summary of the Eagle Ford Formation and the overlying Austin Chalk Formation in selected wells. Understanding paleoredox conditions is of paramount importance to shale gas exploration, since high TOC values are only typically found in sediments deposited where bottom conditions were anoxic or euxinic. Oceanic anoxic events have long been recognized and studied (Schlanger and Jenkyns 1976) and in recent years, much has been written on the use of elemental geochemistry in sediments and water columns as a proxy for depositional redox conditions (e.g. Tribovillard et al., 2006, Turgen and Brumsack 2006, Tribovillard et al., 2008, Negri et al., 2009, Jenkyns, 2010). The key to using major and trace element changes to understand paleoredox in ancient sequences is understanding the geological controls on each of the elements. Principal components analysis provides a quick and effective way to detangle the influences of terrigenous input, carbonate production and authigenic enrichment from sea water on major and trace elements. Vertical and lateral changes in elements associated with authigenic enrichment within the Eagle Ford Formation provide a means to understand temporal and geographic changes in paleoredox conditions, therefore providing important data regarding likely hydrocarbon productivity.
Relative Rock Brittleness
Another important feature of shale gas production is the “fracability” of the formations being drilled. This is controlled by the inorganic and organic mineralogy of the sediments and the rock fabrics. Using the whole rock geochemical data it is possible to define a relative brittleness value for any analysed sample. While this does not provide a quantitative value such as a Young’s Modulus calculation, it does provide a rapid and visual indication of relative brittleness within the formation. This measure can be rapidly determined from core samples as well as from cuttings samples in horizontal wells.
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| Conclusions |
Until relatively recently, the prime purpose of obtaining whole rock inorganic geochemical data for the petroleum industry has been for stratigraphic purposes. However, with increased exploration in shale resource plays, it is rapidly becoming apparent that the same dataset obtained to help refine stratigraphic correlations can be used to; o Determine bulk mineralogy semi-quantitatively
o Determine TOC semi-quantitatively
o Understand temporal and lateral variation in paleoredox conditions
o Determine relative changes in rock brittleness
While the calculations of mineralogy, TOC and brittleness are not as accurate as direct measurements using XRD, LECO or rock mechanics methodologies, the results described here can all be achieved rapidly and at no extra cost from the same ICP-derived data used for chemostratigraphy. Furthermore, the applications for the Eagle Ford Formation can readily be applied to any shale resource play. |
| Acknowledgements |
We are grateful to Pioneer natural Resources for aallowing us to publish this data and to Chemostrat for providing support and allowing us time to preapre the manuscript |
| References |
Hildred, G.V., Ratcliffe, K.T., Wright, A.M., Zaitlin, B.A., Wray, D.S. (in Press) Chemostratigraphic applications to low-accommodation fluvial incised-valley settings; an example from the Lower Mannville Formation of Alberta, Canada. Accepted for publication in Journal of Sedimentary Research, July 2010 |
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