Exploration of Cretaceous and Pennsylvanian Sandstone Reservoirs Utilizing Surface Geochemical Methods

Steven A. Tedesco

Cretaceous and Pennsylvanian sandstones of eastern Colorado are elusive targets that have usually been found by subsurface mapping or random drilling rather than by seismic. The difficulty in exploration is that many channel trends are unpredictable, and when sand is found, it often does not contain petroleum. The normal exploration process of mapping channel trends focuses on the use of previous drilling data which is augmented, in some cases, by seismic. Most exploration plays rely on wells that have indications of Pennsylvanian and Cretaceous sands or channeling. Sometimes these same wells have minor amounts of petroleum present indicating a possible oil accumulation nearby. The use of 3D and 2D seismic can define channel trends but cannot determine the presence or abs nce of petroleum. Seismic may provide clues, but economic costs and data acquisition problems in certain areas inhibit the utilization of extensive surveys. Consequently, wildcat success rates in eastern Colorado have continued to decline.

One solution to channel prediction is the use of surface geochemistry. In recent years geochemistry has gained acceptance as many users of the technology have been able to learn how and where to use it effectively and not attempt to apply it beyond its limitations. Companies that utilize surface geochemistry as part of their exploration program have reported wildcat success rates of 50 to 60 percent (Santa Fe Minerals, Vancouver, 1994 Crown Central Petroleum, 1962). Surface geochemistry's best use is as a screening tool. It provides a way to eliminate areas that have no indication of microseepage and to therefore focus seismic and drilling on areas that do. Consequently, the intent of surface geochemistry is to map the presence or absence of microseepage. Sampling in uniform grids ove large areas along proposed trends allows specific site focus early in the exploration program. In areas where geologic data are sparse, surface geochemistry might help in defining channel trends, identifying areas where sands may contain economic accumulations of petroleum, eliminating areas that do not, and therefore minimizing costs. Examples are presented of the regional mapping of D sand of Cretaceous age and Morrow of Pennsylvanian age utilizing surface geochemistry followed by seismic and drilling. In addition, the integration of surface geochemistry with the regional geology is highlighted. Even though surface geochemistry enhances success, the pitfalls and problems do occur, and they will also be presented.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995