Marine Clastic Reservoirs

This chapter reviews the stratigraphic and paleogeographic setting of shallow marine reservoirs by discussing the sequences in which they occur, the fundamental variables that control them, their geometries, and common occurrences. Many of the concepts of sequence stratigraphy will be discussed as they apply to these kinds of deposits. The development of unconformities, truncation surfaces, diastems, or ravinement surfaces is of primary importance in exploration for, or development of, shoreline and shallow marine reservoirs.

Seismic stratigraphy has developed almost as three separate disciplines. The first, pioneered by Vail and others (1977), is seismic sequence analysis. Sequence analysis is the study of recognizing and correlating regional stratal surfaces to define genetically related rock units that represent discrete chronostratigraphic intervals (Van Wagoner et al., 1988). Seismic data are used extensively in sequence stratigraphy, as primary seismic reflections are generated by time-correlative bedding surfaces, and surfaces that separate sets of contemporaneous depositional systems (systems tracts) (Van Wagoner et al., 1988).

The Amadeus basin is a broad intracratonic depression that formed the setting for the deposition of as much as 14 km of predominantly shallow-marine sediments during the Late Proterozoic and Early Paleozoic (Figs. 3.1, 3.2). The basin, which covers approximately 155,000 km², lies at the center of the Australian craton to the south of Alice Springs. From east to west, along its longest axis, it extends for 800 km. It is one of a number of similar shallow intracratonic depressions that were initiated across the Australian craton in the Late Proterozoic. They are the product of two separate and distinct periods of crustal extension that appears to relate to the breakup of the Proterozoic supercontinent (Lindsay et al., 1987). All these basins contain shallow-marine to non-marine successions, and all appear to have been interconnected through much of their history.

The coasts of North Carolina, South Carolina, and Louisiana are excellent examples of the range of sand-body types deposited along a terrigenous-clastic barrier island shoreline (Fig. 4.1). Submergence during the Holocene and the presence of reworked Holocene and Pleistocene sediment sources resulted in the formation of barrier-island, tidal-inlet, flood- and ebb-tidal delta, estuarine, and complex backbarrier environments. Hayes (1975), Nummedal et al. (1977), and Davis and Hayes (1984) determined that the geomorphic variability of barrier islands and tidal inlets along the southeast U.S. coast is controlled by regional changes in wave regime, tidal range, and tidal prism. In addition, Nummedal et al. (1977) and Hubbard et al. (1979) noted the geomorphic differences among wave-dominated, transitional, and tide-dominated tidal inlets.

The Nile Fan in the southeastern Mediterranean is a large, deep-sea sedimentary fan deposit that started to accumulate in the early Pliocene receiving its material from one primary source—the Nile River. The Nile River is 6,800 km long; it drains a basin of 3 million km², and has an average annual water discharge of 86 billion m³ (Said, 1981). Prior to construction of the Aswan High Dam in 1964, the Nile discharged approximately 120 million m³ of sediment annually to the southeastern Mediterranean basin (Said, 1981). Due to the Aswan High Dam and the intensive irrigation system in the Nile Delta area, the present rate of sedimentary discharge is negligible (A. Golik, personal communication, 1990). The Nile has constructed a large alluvial plain, covering an area of 22,000 km². Herodotus, in the fifth century B.C., was the first to apply the term “delta” to this plain of triangular shape.

The diagenetic processes that control the distributions of cements and porosity within reservoirs are poorly known. However, two lines of reasoning suggest that cement and porosity distributions should reflect both the geometry of the depositional units and the geometry of faults and folds. First, diagenetic processes are stabilization processes. That is, the reactive components in the sediments interact with the subsurface environment to form more stable compounds. Because the original distribution of reactive components was controlled by the depositional systems, some spatial relation between the depositional architecture and the diagenetic products might be expected. Second, many of the processes that precipitate cement or modify porosity involve dissolution, transport, and re-precipitation by subsurface waters. Hydrology is strongly controlled by depositional architecture and tectonic structure, so the distribution of dissolution and precipitation products must also be controlled by these factors.

The late Paleocene-early Eocene Wilcox Group represents the first major, regional clastic wedge built over and beyond the Cretaceous carbonate shelf edges along the Texas Gulf Coast. The Wilcox produces hydrocarbons along a strip paralleling the coastline from the Mississippi-Alabama state line to the Mexico border. This sequence also yields fresh water, lignite, ceramic clay, and industrial sand, and is a potential geopressured geothermal reservoir.

The Potiguar Basin is located in northeastern Brazil and occupies an area of about 40,000 km², distributed both onshore and offshore (Fig. 8.1). It is one of the Brazilian marginal basins originated by Africa-South America rifting in the Early Cretaceous. In recent years, the onshore portion has undergone intense exploration, resulting in the discovery of several small oil fields that produce from Lower and middle Cretaceous rocks. The mid-Cretaceous is represented by the clastics of the Açu Formation, which comprises a lower fluvial unit and an upper, transitional unit known as the Mossoró member.

The discovery of the Canto do Amaro oil field in November 1985 is a milestone in the exploratory history of the Potiguar Basin, which has turned out to be the most productive onshore oil field of Brazil and has opened new prospects in an area previously considered to be of minor importance. The Canto do Amaro oil field is located in the onshore part of the Potiguar Basin, Rio Grande do Norte State, in the extreme northeast corner of Brazil (Fig. 9.1). This field has nineteen producing zones, all of them in sandstones of the Upper Cretaceous Açu Fm. The Açu Fm in the Canto do Amaro area is about 650 m thick, and shows, from the base to the top, a transition from coarse sandstones deposited by braided rivers, through medium sandstones deposited by meandering rivers, to fine sandstones deposited in a marginal marine system. The fine sandstones grade upward into an argillaceous zone and then into the shallow-marine limestones of the Jandaira Fm, making up a transgressive megasequence (Fig. 9.2).

The Permian Basin of western Texas and southeastern New Mexico is a mature petroleum exploration province. The western portion of this basin complex is the Delaware Basin (Fig. 10.1). This subsurface study focuses on the uppermost Guadalupian sandstone, the Ramsey member, of the Bell Canyon Formation (Delaware Mountain Group) where it subcrops near the center of the Delaware Basin (Fig. 10.2).

The Inglewood Field is located in the western Los Angeles Basin, southern California, about 8 mi (12.8 km) southwest of downtown Los Angeles (Fig. 11.1). It was discovered in 1924 by the Standard Oil Company of California, and was completed in the Pliocene Vickers zone. Subsequently, seven additional productive zones have been discovered and are producing in the field.

Deep-water sands form economically important hydrocarbon reservoirs in many parts of the world. Although they have been studied extensively from a traditional, somewhat qualitative perspective, quantitative reservoir characteristics are poorly understood, and often are not described in a format suitable for reservoir engineering applications. Like other types of sands, heterogeneities of deep-water sand reservoirs can be described at four scales (terminology after Krause et al., 1987): microscale (grains and pores), mesoscale (near well bore), macroscale (interwell), and megascale (field-size).