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Understanding Depositional System and Stratigraphic Framework for Improved Deep-Water Reservoir Definition

Chen, Jiajie 1; Laguros, George 1; Petro, David 1; Kulkarni, Madhav 1; Bastidas, Rodrigo 1; Carroll, Johnny 1; Caldwell, Don 1
1 Marathon Oil Company, Houston, TX.

One of the critical steps in deep-water reservoir characterization is defining a stratigraphic framework that is realistic and applicable for reservoir model building. Describing deep water depositional systems has been a great challenge to the geoscience community due to the complex facies architecture in turbidite systems, the limitation of seismic resolution, and the possible stratigraphic interference with salt structures. Therefore, resulting reservoir models can either be too simple so that no meaningful flow patterns are described, or geologically so complex that no simulators can really handle them.

In this study we developed a workflow to adequately model the depositional system and construct the stratigraphic framework for a Gulf of Mexico turbidite reservoir. Using well logs, paleo and geochemical data, seismic and cores, we first applied principles of sequence stratigraphy to define the third and fourth order bounding surfaces, or the limits of reservoir zones, which are correlable throughout the entire field. These surfaces separate the stratigraphic intervals into reservoir units that reflect evolution from the regional flooding zone (trangressive stage) at the base of each cycle to the sandy zone (regressive stage) on top of each cycle. This contributes to vertical reservoir disconnectivity that is also observed from pressure analysis. We then conducted iterative seismic inversions for acoustic impedance (AI) using the bounding surfaces. Based on the AI volume, we identified secondary downlapping surfaces that are internal to the third order cycle boundaries. These downlapping surfaces represent the intermittent ponding of sand-rich turbidite flows against the salt interface at higher frequencies compared to the third order cycles, forming the complex facies. Further, we integrated all available data sets, including pressure trends, to define reservoir compartments and estimate oil-water contacts.

The resultant stratigraphic framework model reflects a full inter-disciplinary integration of available data sets. Using the model we were able to predict reservoir tops within 20 feet during our delineation drilling program. The framework model, when populated with reservoir properties such as porosity and net-to-gross, allows us to objectively estimate the resource base, and conduct reservoir simulation in order to guide our upcoming development program.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009