Seismic Stratigraphy of the Attahady Field Concession 6, Sirte Basin, Libya

The Attahaddy Field is a giant gas field discovered in 1964. It covers approximately 43,300 acres. Several exploration and outpost wells were drilled, displaying very good producibility in the field. This encouraged the company to drill numerous development wells. The total number of wells that have been drilled to date is 16, however, the development work is still continuing at the present time. Seismic coverage over the Attahaddy Field comprises an area of approximately 200 km2, of 12-30 and 48 fold Vibroseis seismic data acquired in 1980, 1983, 1984 and 1985 in the Concession 6 Area. The addendum incorporates an additional 270 km2 of 30 fold Vibroseis data recorded in 1987. The final interpretation presented in this study was carried out using only the 1987 data. This is due to the unresolvable seismic time misties between the different vintages caused by the use of different acquisition and processing parameters. The 1987 data, which provides adequate coverage for mapping, forms a seismic grid with the lines oriented in a NW-SE and NE-SW direction with approximately 2.5 km line spacing. The stratigraphy of the Attahaddy Field starts at the top from the undifferentiated Oligocene-Miocene which consists of a mixture of deep-shallow marine shales and limestones with some interbedded clastic deposits of loose sand, which occurred during the transgression and regression of the sea during that time. Deeper in the section, the sedimentation of the Lower Eocene is dominated by carbonates, which have good porosity. It is believed that due to the lack of the source and cap rocks, no hydrocarbon accumulation has taken place. The Palaeocene deep marine deposits (a very thick section of green shale) are believed to be the source rock for the underlying structural traps in most of Concession 6. In addition, a lot of problems have been encountered as the shale was penetrated, due to the stickiness and softness of the shale, resulting in the drill bit sticking during drilling, and thereby slowing the drilling time. The Palaeocene shale has been subdivided into two seismic sequences, in which the lower part is characterized by layers truncated at the boundary with the upper part. This seismic stratigraphic subdivision is a new result from the present study. The upper part of the subdivided sequence is characterized by parallel layers that show a concordant relation to the upper bounding surface. The Upper Cretaceous units consist mainly of sandy limestone and occasionally in some parts of the field consist of calcareous sand, underlain by the deep marine deposits of the Socna shale/limestone, which is absent in some places of the field as a result of the intra-Cretaceous unconformity. As a result of the intra-Cretaceous unconformity, the deep marine deposit (Heira shale) of Palaeocene age was laid down above the Upper Cretaceous shallow carbonate deposits (Zmam Formation) which are separated from the Upper Cretaceous clastic deposits (Bahi Formation) by the early Cretaceous unconformity. The thick underlying Cambro-Ordovician metamorphic section consists of highly fractured, massive, dense quartzitic sandstones and quartzite which form the hydrocarbon pay zone of the Attahaddy Field