James A. Miller

Structural Control of Jurassic Sedimentation in Alabama and Florida

The extent and thickness of Jurassic strata in Alabama and Florida were mapped as part of a U.S. Geological Survey regional geohydrologic study. Rocks equivalent to the Werner, Louann, Norphlet, Smackover, Haynesville, and Cotton Valley sequences of Mississippi and Arkansas have been recognized and, except for the Werner equivalent, are herein extended into panhandle Florida. The predominantly fluvial Jurassic sequence in Alabama and Florida was interrupted at least twice by barred-basin conditions that resulted in the deposition of extensive evaporite deposits, and at least once, during deposition of the Smackover Formation, by a major marine transgression. The updip limits and isopach map trends of the Jurassic units mapped appear to have been controlled by a major nort west-trending, right-lateral wrench fault--the dominant master shear in a regional wrench-fault system active throughout Jurassic time.

Reservoir Geology of Devonian Carbonates and Chert--Implications for Tertiary Recovery, Dollarhide Field, Andrews County, Texas (1)

Devonian strata of the Thirtyone Formation at Dollarhide field, Andrews County, Texas, originally contained 144 million bbl of oil in place, with 18.6 million bbl produced by primary recovery, 39.3 million bbl projected to be produced by secondary recovery (waterflood), and 27.4 million bbl projected to be produced by tertiary recovery (CO[2] flood). Devonian strata contain five main lithologies (from bottom to top): (1) carbonate-chert mudstone deposited in a basinal environment; (2) burrowed chert-dolomite (10-35 ft or 3-11 m thick), which accumulated as siliceous spiculitic wackestone and carbonate mud in a low-energy slope environment; (3) laminated tripolitic chert (0-65 ft or 0-20 m thick) deposited as a sponge spicule sand in submarine channels and fans on the slop ; (4) bioclastic limestone (47-92 ft or 14-28 m thick), which accumulated as crinoidal grainstones in moderate to high-energy, outer shelf environments; and (5) upper dolomite (50-90 ft or 15-27 m thick) with dolomitized peloidal grainstones deposited in high-energy, shallow subtidal to intertidal environments, and thin chert beds deposited in supratidal environments. Deposition of the 180-ft-thick (55 m) reservoir section occurred during a basinward progradation of shelf and slope facies. Porosity within Devonian strata is controlled mainly by depositional facies. Porosity occurs in two stratigraphic zones separated by the tight bioclastic limestone. The lower porosity zone (laminated tripolitic chert and part of the burrowed chert-dolomite; 0-100 ft or 0-30 m thick) is relatively homogeneous (not stratified), with high porosity (commonly 25-35%), moderate permeability (5-20 md), and short fractures causing elevated permeabilities near the bottom. The lower reservoir is thick in fan-shaped and elongate areas where sponge spicules were deposited on submarine fans and in submarine channels. The upper porosity zone (0-90 ft or 0-27 m thick) is within the upper dolomite and is heterogeneous, with stratified porosity that bifurcates, coalesces, and pinches out in several di ferent intervals. The relative homogeneity and moderate permeability of the lower reservoir should help achieve a uniform sweep during CO[2] flooding. Higher permeabilities near the base of the lower reservoir should minimize CO[2] override, thereby improving sweep efficiency. The thinner, more heterogeneous, less continuous, more stratified, more permeable pay in the upper reservoir will result in more rapid and less uniform CO[2] sweep.

Petrology of Eocene Rocks, Southeastern Georgia Coastal Plain: ABSTRACT

Study of cores from a US Geological Survey test well in Wayne County indicates that Eocene strata represent an overall shallowing-upward, clastic-carbonate sequence. The 1397-ft (426-m) Eocene section is divided into three units: unnamed lower Eocene rocks, middle Eocene (Claibornian) Lisbon and Avon Park Formations, and upper Eocene (Jacksonian) Ocala Limestone.

Depositional and Diagenetic Facies, Smackover Formation, Chunchula Field, Alabama: ABSTRACT

The Chunchula field lies on the northeastern edge of the South Mississippi platform and produces from dolomitic carbonates of the Smackover Formation. The Smackover section overlies the subaerial to marine Norphlet Sandstone and itself represents a general transgressive-regressive sequence of shallow-marine to supratidal facies similar to those found on the Great Bahama Bank today. The Chunchula carbonate section is composed of at least three major units. The basal interval is the lowermost Smackover section and is composed of medium to coarsely crystalline dolomite and its upper boundary seems to be marked by a significant disconformity. The second unit is interpreted as upper Smackover and is composed of medium to coarse-grained dolomite in the central and western parts of the field, but becomes predominantly limestone along the northern and eastern edges of the field. The uppermost part of the carbonate section is a finely crystalline dolomite that represents part of a sabkha sequence and probably belongs to the overlying Buckner evaporite section. Porosity development is restricted to the dolomitic units and seems to be preferentially associated with paleotopographic highs. The best reservoir intervals are composed of intercrystalline dolomite and pelmoldic porosity and have their maximum development in the southeastern part of the field. Carbon and oxygen isotopes and strontium ion concentration data suggest that fresh or brackish fluids have played some role in the development of porosity in the Smackover carbonates. End_of_Article - Last_Page 607------------

Biostratigraphy of Early and Earliest Late Cretaceous Ostracoda from Peninsular Florida: ABSTRACT

Deep wells in central and southern Florida have yielded 68 species of marine and brackish-water Ostracoda of Early Cretaceous and earliest Late Cretaceous ages. About 40% of the ostracods are referable to described species. The rock sequence is more than 2,250 m thick. A partly oolitic limestone facies of the Washitan Stage (early Cenomanian-late Albian) contains 17 species, of which 15 are restricted to the unit. Brackish-water, and perhaps freshwater, as well as marine Ostracoda are represented. Pre-Washitan Cretaceous rocks of peninsular Florida are principally massive, interbedded carbonate rocks and evaporites and thin shales. Ostracods occur chiefly in the shales. Of 23 species in the Fredericksburgian Stage (middle Albian), nine are restricted to the unit; marine and a few brackish-water species are represented. Trinitian Stage (early Albian-late Aptian) ostracods are represented by 21 species, of which seven are restricted to that unit; several are brackish-water forms. Coahuilan Series (early Aptian-Neocomian) rocks contain 35 species, of which 25 are confined to that unit; several are brackish-water types. The environment represented by the ostracod populations is mainly that of an open-marine shelf bordered by partly brackish-water lagoons. Trinitian Stage rocks contain representatives of a few species which indicate outer-shelf or slope environments. The population as a whole has strongest affinities for Cretaceous species of the Gulf coastal or Atlantic coastal United States. Several of the Coahuilan and Trinitian species show relationships to European and to South American forms. Few such relations are shown by Fredericksburgian and Washitan species. End_of_Article - Last_Page 536------------