Depositional Sequences in Pensacola Clay of Southwest Alabama and Their Significance in Petroleum Exploration: ABSTRACT

Abstract

ABSTRACT In southwest Alabama, the subsurface Pensacola Clay of Miocene age overlies Tampa Stage sediments of early Miocene age and is overlain by Miocene coarse clastics. The Pensacola Clay consists of greenish-gray to light-olive-gray, slightly calcareous, glauconitic, micaceous, fossiliferous, silty to sandy clay containing beds and lenses of sand, some of which produce gas. Thickness of the Pensacola Clay increases southwestward. The Miocene coarse clastics are micaceous, locally carbonaceous and fossiliferous, and contain numerous beds of small mollusk shells. Within the Pensacola Clay and the overlying Miocene coarse clastics three separate progradational marine sequences may be recognized. Generally, sediments within each sequences coarsen upward. The lowermost sequence, the Amos depositional sequence, contains the gas-productive Amos sand. The middle sequence, the Escambia depositional sequence, has the Escambia Sand at its top. The uppermost sequence includes the upper member of the Pensacola Clay, which contains the gas-producing Meyer sand, and the interfingering Miocene coarse clastics. Foraminifera found in the Pensacola Clay are indicative of outer to inner neritic environments. Foraminiferal species number and diversity, as well as number of planktonic species, generally decrease upward within each depositional sequence indicating an increasingly restrictive marine environment and shallowing of the seas. The presence of Globorotalia fohsi fohsi, G. fohsi robusta, G. fohsi lobata and G. praemenardii in the Amos depositional sequence indicates a middle Miocene age for the sequence. A late Miocene age is indicated for the Miocene coarse clastics by Rangia (Miorangia) microjohnsoni. The three depositional sequences appear to correlate with the TM 2.2, TM 2.3, and TM 3.1 third-order cycles of sea-level change as proposed by Vail et al. in 1977. Isopach and net sand maps of the different depositional sequences are useful for petroleum exploration. Comparison of such maps for the Amos depositional sequence indicates that in Baldwin County Amos sands (the most productive of the Miocene sands) occur where the depositional sequence is locally thinker, probably as a result of postdepositional compaction of clays surrounding sand bodies. Sands found at the top of the Amos depositional sequence are often productive because these sands have been transgressively reworked and are overlain by marine source beds.