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Where's the Sweet Spot? Approaches to Exploration in Resource Plays, with Particular Reference to Mapping TOC

Katy Jensen-Doescher and Brian Lock
[email protected]

When a company is deciding on becoming active in a resource play, the principal exploration objective is to select 'sweet spots' where investment is most likely to be profitable. The approach is rather different from that involved with conventional plays. Sweet spots are areas where a combination of factors are optimized – thickness of the resource unit, appropriate level of maturity, type of kerogen, brittleness of the rock, stress patterns and natural fractures and their orientation, and, particularly critical, level of total organic carbon (TOC). While data from cores which have been analyzed are very valuable, the quantity of such data available is generally limited, even if the company joins a consortium. Vertical wells with released well log suites are likely to be much more common, and methods for estimating TOC from those well logs are therefore very important. Since several methods have been devised, using various combinations of curves, there is great value in studies within the particular play that permit relative evaluation of the utility of different log-based approaches. The objective of this study has been to provide this type of evaluation for a central area within the Eagle Ford resource play. A study well was selected for which conventional core was available. Rock Eval pyrolysis provided, amongst other critical data, measures of TOC at intervals through the Lower Eagle Ford (Transgressive Systems Tract). Well logs from the same well were used to determine estimates of TOC from the same intervals, using the Delta Log R and GR and Rt methods. The GR and Rt Method proved superior to the Delta Log R Method. The GR and Rt Method is not as robust as anticipated. Anomalous TOC results resulted from the calculations where the GR value is over 100 or the Rt value is over 16. These limitations imply that the GR and Rt Method is only applicable for shale and shale-like plays where the hydrocarbons are part of the rock matrix as well as for plays that do not have a very high clay component that would otherwise result in a high GR reading. The conclusions reached during this study may not be appropriate in other areas of the Eagle Ford as well as in other unconventional reservoirs, and testing is needed in a greater variety of areas and other plays.

AAPG Search and Discovery Article #90167©2013 GCAGS and GCSSEPM 63rd Annual Convention, New Orleans, Louisiana, October 6-8, 2013