Deltaic Environment Reservoir Types and Their Characteristics

Abstract

Processes responsible for creating end members of a continuum of deltaic Processes responsible for creating end members of a continuum of deltaic sedimentation and the characteristics of reservoirs resulting from these processes are presented. Deltas tend to be either high-energy sand deltas processes are presented. Deltas tend to be either high-energy sand deltas or low-energy mud deltas. In either case, reservoirs generally are confined to the bar-fringe sands and/or to distributary channel sands. Frequently, reservoir quality is discontinuous. Introduction The need to increase ultimate recovery requires that the engineer and earth scientist work together to depict accurately the character and geometry of a reservoir. This synergistic relationship results in a better understanding of the quality, distribution, and continuity of the reservoir and its contained fluids. As most known accumulations of oil and gas trapped in terriginous (land-derived) sediments are contained in sandstones deposited in deltas, there is an increasing need to understand these reservoir types better. Knowledge ofthe type of reservoir potentially available,the distribution and quality of pore space in terms of porosity, permeability, and capillary pressure properties, and porosity, permeability, and capillary pressure properties, andthe location of barriers to flow, both internal and external, should enhance greatly the ultimate recovery from this important source of oil and gas. The need for information about these reservoirs prompted this summary paper, which is based solely on prompted this summary paper, which is based solely on and distilled from information available in published sources. In view of the spectrum of environments in which deltas, both ancient and modern, are known to exist, it would be foolhardy in this summary to attempt a review of the myriad types available. Therefore, only the processes responsible for creating the end members of a processes responsible for creating the end members of a continuum of deltaic sedimentation and the characteristics of reservoirs created by these processes are presented. With this information and enough data, the presented. With this information and enough data, the inquiring geological/engineering team should be able to build on this foundation and depict accurately any specific reservoir in the deltaic environment. Principles of Deltaic Sedimentation Principles of Deltaic Sedimentation Deltas are abundant in the geologic record and are known to exist in ancient, recent, and modern settings. For example, 32 large deltas are forming at this time (Fig. 1). Countless others are in various stages of growth throughout the world. In 1912, Joseph Barrell described a delta as "a deposit partly subaerial (above sea level) built by a river into or partly subaerial (above sea level) built by a river into or against a body of permanent water." Three prerequisites for deltaic sedimentation are apparent from this apt description (Fig. 2):drainage basin, a source of sediment,river, to move the material, andreceiving basin, to store and rework it. As a delta forms, the outer and lower parts are constructed below water, and the upper and inner surfaces become land "reclaimed" from the sea. Therefore, even though deltas are visualized commonly as subaerial, their subaqueous portions are truly significant. A delta is created by an iterative process of progradation (built outward) in which sediment literally builds on progradation (built outward) in which sediment literally builds on itself until a particular pathway is no longer available, and another delta forms in a different location. Thus, any given delta is a composite reflecting conditions at hand from the time of inception until ultimate abandonment of a particular deposition center. JPT P. 1538