Geological Significance of Abnormal Formation Pressures

Abstract

The significance of the distribution of abnormal pressure in the Bigenerina A of the Gulf of Mexico basin area offshore Louisiana is its persistent relationship to such mappable geological characteristics as its regional correlation with low sand content and its confinement to the continental slope. Introduction Many companies and individuals have investigated abnormal formation pressures; however, few studies have been published relating the geology to this phenomenon. The first and most comprehensive study phenomenon. The first and most comprehensive study of the geologic aspects of abnormal pressures was published by George Dickinson in 1953. Although he published by George Dickinson in 1953. Although he considered engineering, production, and drilling problems, his major contribution was the recognition problems, his major contribution was the recognition that stratigraphy controls the regional distribution of normal pressure. Abnormally high formation pressures are common in the post-Cretaceous sediments of the Gulf of Mexico basin, particularly so in southern and offshore Louisiana. Wherever thick clays are deposited rapidly, as in the Gulf basin, interstitial water is likely to be trapped and isolated from communicating with the surface. In this situation the sediment cannot compact and the contained water is subjected not only to hydrostatic forces, but also to the weight of newly deposited sediment. This results in a formation with abnormal fluid pressure. Our purpose is to illustrate the geological significance of abnormal pressures by relating their occurrence to certain geological conditions within a specified time-stratigraphic unit. In addition, it is pertinent to consider the occurrence of oil and gas within pertinent to consider the occurrence of oil and gas within the unit. The designated stratigraphic unit is the Bigenerina A, shown on Fig. 1. The area studied is the Louisiana offshore west of the bird-foot delta (Fig. 2). By definition, normal formation pressure is usually equated with hydrostatic force. For the Gulf Coast area the typical pressure gradient, 0.465 psi/ft of depth, is equated to approximately 9.0 lb/gal drilling mud weight. Hence, formation pressures equivalent to 9.0 lb/gal mud are considered normal. Any pressure exceeding this is abnormal. However, for our purposes, any pressure above 12 lb/gal mud weight equivalent is considered abnormal because any formation pressures exceeding this commonly cause drilling problems. Some of these problems are lost circulation, lost hole, stuck drill pipe, blow out, and casing setting. Any one of these or others associated with abnormal pressure will increase drilling costs. Stratigraphy and Associated Abnormal Pressure The relationship between abnormal pressure and stratigraphy is a striking one. Although recognition of this relationship is not new, a few comments about it are warranted. Abnormal pressures are first encountered in the intertonguing sands and shales just below the base of more massive and continuous deltaic sands. To develop abnormal pressures the shales usually must be over 200 ft thick. The intertonguing sand shale facies forms down slope from the deltaic facies, and therefore, as a prograding sequence, tends to rise stratigraphically in a basinward direction. JPT P. 961