Roger M. Slatt
Funded by the AAPG Foundation through the J. Ben Carsey Memorial Endowment
Outcrop/Behind Outcrop Characterization of Deepwater (Turbidite) Petroleum Reservoir Analogs: Why and How
Increased emphasis on exploration for deepwater (turbidite) reservoirs during the past 25 years has resulted in many discovered fields. The current trend is for companies to fast-track field development, with minimum drilling, no workovers or intervention, increased use of subsea tiebacks, optimal individual well rates and ultimate recoveries, and expanded perforated intervals. To accomplish this requires a good understanding of the architecture of these reservoirs, and early prediction of their production performance based upon reservoir simulation.
Geologic models that are used for simulation are often built using a limited number of appraisal wells, which may hinder accurate bed correlations, and 3D seismic, which may not resolve sub-seismic scale geologic features that control production, and thus the economic viability of the project.
During the past few years, quantitative characterization of outcrops has supplemented subsurface data in the model-building process. However, to adequately document important reservoir properties at the wellbore, interwell and reservoir scales requires studying large, continuous outcrops, preferably in 3D. Unfortunately, such outcrops are rare, so that incomplete characterizations of reservoir analogs are normally the end product. In addition, mud-prone, thin-bedded deposits, which form a major portion of global deepwater reservoirs, normally do not provide as good an outcrop exposure as do sand-prone, thicker-bedded deposits. Thus, accurate characterizations of the thin-bedded deposits are difficult to achieve.
Incomplete models, or use of the wrong model for simulation, have given rise to one school of thought that regards outcrop data as being of limited value.
Despite these shortcomings, a second school of thought considers quantitative outcrop characterization as providing valuable information, particularly at the critical interwell scale. Information on shale- and sandstone-bed continuity, vertical connectivity, internal geometry and hierarchy of architectural elements, as well as stratigraphic variations in permeability are all important input attributes for understanding well performance, as well as for geostatistical modeling, upscaling geologic parameters, and providing well log recognition criteria for predicting continuity away from a wellbore.
Several tools and techniques are used to characterize outcrops.
- Photomosaics and immersive 3D visualization photoimaging provide a means of capturing architectural information in an electronic format amenable to overlay onto seismic sections or between wells.
- Logging, coring, and collecting seismic reflection data behind outcrops, as well as collecting gamma ray, sonic velocity, and permeability profiles along outcrop faces provide data that can be directly compared with subsurface well logs and seismic.
- Ground-penetrating radar (GPR) provides the only means currently available for continuously imaging small-scale features behind an outcrop face.
These techniques have been successfully applied to outcrops of three mud- and mud/sand-prone deepwater (turbidite) deposits. Studies of the Upper Cretaceous Lewis Shale (southwest Wyoming) have provided:
- a high-frequency sequence stratigraphic framework for detailed correlation purposes;
- borehole image criteria to distinguish sheet from leveed-channel sandstones;
- quantitative 2D bed continuity and connectivity information;
- GPR images of the complex nature of the critical boundary between channel and levee/overbank deposits; and
- a framework for development of a partial 3D model of sheet and leveed channel deposits at reservoir scale.
Studies of the Lower Pennsylvanian Jackfork Group (Arkansas and Oklahoma) have provided:
- a sequence stratigraphic framework;
- interwell-scale 3D geologic models of channel and sheet sandstones, including one which has been subjected to ‘reservoir’ simulation;
- quantitative bed continuity data at the interwell scale; and
- GPR imaging for stratigraphic detail and paleocurrent analysis.
Studies of the Late Miocene Mt. Messenger Formation (New Zealand) have provided:
- a sequence stratigraphic framework;
- dipmeter criteria to distinguish channnel, proximal- and distal-levee deposits;
- detailed thin-bed continuity and connectivity data; and
- internal architecture of levee and channel-fill strata from outcrop and high-resolution seismic images.
Although complete 3D outcrop geologic models of these deepwater (turbidite) deposits at reservoir scale are still elusive, the results provide important information that is directly applicable to interpreting and predicting performance of analog reservoirs. Characterization of outcrops using the combination of tools and techniques described here should continue with the ultimate goal of providing the necessary quantitative information to help guide fast-track development of deepwater (turbidite) reservoirs.
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