Seismic expression of structural style in Norphlet formation, offshore Alabama

Abstract

During the past several years, the Jurassic Norphlet formation in offshore Alabama has been the focus of active exploration and development operations. Since the 1979 discovery of deep gas [greater than 6096 m (20 000 ft)] in Norphlet sandstones which contain estimated reserves of several trillion cubic feet, six Norphlet fields have been established in Alabama state waters and an additional six fields have been established in Federal Outer Continental Shelf (OCS) waters in offshore Alabama. We describe, using seismic data, the structural style associated with the Norphlet formation in offshore Alabama. More than 563 line‐kilometers (350 line‐miles) of multifold common‐depth‐point (CDP) seismic reflection data in Mobile Bay and offshore Alabama have been analyzed, interpreted, and mapped. The Lower Mobile Bay‐Mary Ann natural gas field provides an excellent seismic case study for the structural style in the deep Norphlet play. The field may be used as a geophysical exploration model for other Jurassic structures in offshore Alabama and the central and eastern Gulf of Mexico. Interpretation of the seismic data and maps indicates that Norphlet structures in offshore Alabama are predominantly east‐west trending, low‐relief, broad, elongate anticlines. The Lower Mobile Bay fault trend associated with the anticlines consists of pull‐apart, listric, normal faults characteristic of salt‐detachment structures. Many of these faults exhibit small‐scale growth. Salt thickness ostensibly increases from Mobile Bay to offshore Alabama and is exemplified by the development of a sequence of various structures typically associated with basinward increase of salt. Offshore Alabama structures may be classified as early horizontal phase or pillow‐stage features. Strata above the Haynesville seismic marker appear to be relatively flat, indicating early salt movement in the area. Small downbends associated with salt withdrawal exhibit thickening in the Haynesville‐Smack‐over section and are further complicated by normal faulting. The preponderance of the data suggests that the structures containing the large gas accumulations in the Norphlet formation in offshore Alabama are the result of salt tectonism.