ABSTRACT

The petroleum geologist's search for energy sources leads him to prospective areas where he conceives structural or stratigraphic traps. This is only the beginning, however, for in order to understand and delineate the potential of an area one needs a clear and concise concept of the depth -- burial -- migration sequence of the prospect. In an effort to clarify and relate this concept to future prospects, the writers have chosen to analyze Old Ocean Field.

An explanation is offered for the absence of significant hydrocarbons in pre-F-21 sands. The F-21 is local nomenclature for a producing sand body found approximately 300 feet below the top of the Lower Frio.

One gas-condensate reservoir (F-21) and one oil reservoir (F-12) with a sizeable gas cap are analyzed by following their structural development from the time of earliest closure to the completion time of hydrocarbon migration. This is done through the presentation of a series of cumulative isopach maps.

In the case of the F-21 reservoir, a small anticlinal trap was available to migrating hydrocarbons as early as the time of deposition of the F-19 sand. In the instance of the F-12 reservoir, closure was established by the time of deposition of the Nodosaria blanpiedi marker. Migration could have started this early, provided a supply of hydrocarbons was available.

Based on the size of the traps then available, the depth of burial at that time and the associated pressure -- volume -- temperature relationships, it was deduced that accumulation in both reservoirs could have been completed by the beginning of Miocene deposition, but not much earlier. This time coincides approximately with cessation of movement on the principal fault.

The structure was located in an area of drainage that was large enough and sufficiently rich in hydrocarbon source rocks to provide the known reserves. It is further concluded that a trap existed at such time as physical and chemical conditions permitted release of oil and gas from the source material, and they became free to migrate.

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