David K. Hobday

Frio Formation of Texas Gulf Coastal Plain: Depositional Systems, Structural Framework, and Hydrocarbon Distribution

The Frio Formation (Oligocene-Miocene) is one of the major Tertiary progradational wedges of the Texas Gulf coastal plain and has yielded nearly 6 billion bbl of oil and 60 tcf of gas. The Frio, and its updip equivalent, the Catahoula Formation, consists of deposits of two large fluvial and associated deltaic systems, centered in the Houston and Rio Grande embayments. Structural history in the Houston embayment is dominated by syndepositional deformation of underlying Jurassic salt; mobilization of thick, undercompacted prodelta and slope muds characterized the tectonic evolution of the deltaic sequence in the Rio Grande embayment. These two major deltaic depocenters are separated by a vertically stacked, strike-parallel coastal barrier and strand-plain system. Underlying interbedded, and transgressive shelf, prodelta, and continental slope mudstone sequences provide principal source and sealing facies. Sparse organic geochemical data, regional thermal gradients, and distribution patterns of hydrocarbons show that large volumes of oil and gas have likely been generated within and effectively expulsed upward and landward from thermally mature, normally to moderately undercompacted sequences of bounding mudstone facies. All Frio depositional systems contain major, geologically defined, hydrocarbon-producing plays. Oil and gas field productivity data show a log normal frequency distribution for all but the largest fields. Per volume productivity, production styles, and types of hydrocarbons within each of the ten recognized plays reflect available source rock types, differing thermal and compaction histories, and variable reservoir and trap configurations that characterize each depositional system. Systematic distribution patterns of both physical and chemical properties of produced hydrocarbons can be related to source facies, regional thermal regime, and post generation modification by (1) continued maturation, thermal cracking, and deasphalting, (2) migration and attendant chromatographic separatio , and (3) bacterial alteration.

Terrigenous Shelf Systems

Terrigenous shelves include both epeiric (epicontinental) platforms and continental shelves, with a mantle of land-derived sediments, as opposed to biogenic and chemical precipitates. Epeiric platforms are broad, shallowly inundated continental areas. Modem examples such as the North Sea, Hudson Bay, and Gulf of Carpentaria are small by comparison with many of their ancient counterparts. Continental shelves are submerged continental margins, dipping very gently from the outer edge of the shore zone to a depth, generally between 300 and 800 ft (100 and 250 m), at which there is an abrupt increase in slope. If the shelf break is not well defined, the shelf is arbitrarily confined to depths shallower than 200 m (650 ft) (Bates and Jackson, 1980). Present-day shelves have a complex depositional and erosional evolution which commenced in the Mesozoic (Swift, 1969).

Slope and Base-of-Slope Systems

Slope and base-of-slope depositional systems (simply called slope systems for brevity) occur in relatively deep water beyond the shelf break. However, the meaning of “deep” depends upon basin type and tectonic context. On modern continental margins the shelf break typically lies at depths of 90–180m (300–600ft), but in intracratonic, foreland, and continental pull-apart basins the depth may be less. Furthermore, our concept of the shelf edge and slope setting is biased by our Holocene perspective. Contemporary continental margins lie at depths that increased abruptly during rapid glacioeustatic sea-level rise of about 120m (400ft). Pre-Pleistocene shelf edges probably had a broad, but somewhat shallower depth range; in many basin settings, delta, alluvial plain, or fan systems prograded directly into deep water for extended periods of geologic time, and a distinct shelf and basinward shelf edge were generally absent.

Fuel-Mineral Resource Base

Most of the world’s energy and groundwater resources are in sedimentary rocks, and any procedure that helps to categorize, understand, and predict the external geometry, internal architecture, and other properties of sedimentary rocks can contribute to more efficient discovery, exploitation, and resource management. The depositional systems approach that we advocate involves rigorous three-dimensional characterization of lithological units and analogy with modern depositional environments, taking into account preservation potential and modifications that occur with progressive burial. Where subsurface information is fragmentary or incomplete, identification with a particular depositional system permits extrapolation beyond the data base and anticipation of lithofacies in unexplored tracts. This procedure often proves invaluable in preliminary assessment of prospectivity and in ranking frontier or immature exploration acreage.

Shore-Zone Systems

The shore zone, excluding deltas, comprises the narrow, high-energy transitional environment that extends from wave base, commonly at about 10 m (35 ft) to the landward limit of marine processes (Fig. 6.1). Although the shore zone is a narrow, linear zone, shorelines migrate over time to leave a record of widespread shore-zone deposits, with considerable bearing on the distribution of hydrocarbons, coal, uranium, and other mineral resources.

Depositional Systems and Facies Within a Sequence Stratigraphic Framework

Quantitative analysis of sediment supply rate, subsidence and uplift rates, and eustatic sea-level change demonstrates order of magnitude changes in the ratios of these regime variables over timespans of tens of thousands to a few million years. This chapter focuses on the results of this history of change — the punctuated accumulation of depositional systems and their component facies within a Stratigraphic framework defined by surfaces of nondeposition, bypass, or erosion.

Coal and Coalbed Methane

Coal-forming environments have waxed and waned in extent since the mid-Paleozoic, having been associated with a restricted range of environments and hydrologic regimes during relatively brief geologic timespans. The three major episodes of coal genesis were the late Carboniferous and Permian, the late Jurassic through early Cretaceous, and the late Paleocene and Eocene (Haszeldine 1989), occupying paleoclimatic zones from equatorial to near-polar (Fig. 13.1).

Facies Characterization of Reservoirs and Aquifers

A trend in applied facices sedimentology during the late the twentieth century is the shift in focus from hydrocarbon exploration, with its need for reservoir prediction and extrapolation, to detailed reservoir characterization. As many older oil and gas fields reach maturity, they must be redeveloped and advanced production technologies applied if recovery is to be maximized. At the same time, the burgeoning processing power of reservoir simulators requires sophisticated, quantitative description of the three-dimensional distribution of porosity and permeability and of bounding impermeable layers. Emphasis is on the internal facies and bedding architecture of reservoir sand bodies and on accurate quantification of flow properties. Genetic facies analysis is being successfully applied to this process. Well log, core, and, increasingly, three-dimensional seismic data are being used to map, interpret, and quantify reservoir architecture.

Approaches to Genetic Stratigraphic Analysis

One of the most difficult tasks in the application of genetic facies interpretation in resourc exploration, appraisal, and development is the delineation of depositional units of sufficient extent and appropriate scale for analysis. The depositional basin defines the boundaries and general conditions of the accumulation of a sediment pile. Depositional systems, as described in subsequent chapters, provide “meaningful sections” of the basin fill. Their recognition and delineation establish a framework for facies differentiation and mapping, using appropriate process-response models. It is commonly at the facies level that source units, fluid-migration pathways (the basin plumbing), potential hosts or reservoirs, and trapping configurations are sought and dissected.

Depositional Systems and Basin Hydrology

Although a discussion of hydrogeologic processes and basinal flow systems may seem to be a departure from this examination of genetic stratigraphic analysis and energy mineral occurrence, it is included for several compelling reasons.