Abstract Most basins contain facies changes, unconformities with resulting truncated beds, and buried erosional or constructive surfaces such as reefs, hills, channels, barrier sand bars, and other such phenomena—which form the basic requirements for the creation of subtle traps. If folding, normal faulting, thrusting, and the formation of salt ridges and domes are added to the picture of an evolving but continuously filling basin, the resultant structural and stratigraphic patterns become much more complex. However, no matter how complex the history, those stratigraphic relations and lithologic changes which are conducive to the formation of stratigraphic, unconformity, and paleogeomorphic traps remain. When hydrocarbon is expelled (primary migration) by pressure and heat from sediments which contain source material into adjacent reservoir rocks, it migrates through carrier beds (secondary migration) into sealed reservoirs, or traps. As long as the conditions necessary for secondary migration of a substantial amount of petroleum exist, migration will continue along strike and updip until all migrating hydrocarbons are either trapped in the subsurface or have escaped at the surface. As the petroleum moves, it will be captured by all traps—stratigraphic, unconformity, paleogeomorphic, structural, or a combination of these—which are in the path of migration. Because paleogeomorphic, unconformity, and stratigraphic traps are related (1) to older geologic surfaces, (2) to the location of strata on and directly below an unconformity surface, and (3) to lithologic changes within and laterally adjacent to a stratum, it is suggested that, in general, the conditions which produce most subtle traps are present before development of structural traps. If migration of hydrocarbons through a particular region were to take place before structural movements, all petroleum trapped during this early migration would be in subtle traps. Because subtle traps generally are formed as a result of constantly recurring depositional patterns which usually precede, or may be associated with, contemporaneous structural movement, petroleum basins probably contain more subtle traps than structural traps. Although much petroleum has migrated into structural traps, possibly more has accumulated in the earlier (and contemporaneously) formed subtle traps. Because subtle traps probably contain the large undiscovered domestic reserves needed for the future, explorationists must make the purposeful search for such traps an essential and substantial part of their exploration policy.

Abstract Three decades ago, The American Associaition of Petroleum Geologists published the first comprehensive treatment of stratigraphic traps in the well-known volume Stratigraphic Type ; Oil Fields , edited by A. I. Levorsen. Since that 1 time many important advances have been made in our understanding of stratigraphic traps and in our methods of exploring for them. It is entirely fitting, therefore, that this volume on Stratigraphic Oil and Gas Fields should begin with a section on geologic methods. When Editor-in-Chief Robert E. King asked me to arrange for and organize this section, I invited 12 of the country’s specialists in methods applicable to stratigraphic-trap exploration to join me in this effort. The result was the following nine articles which, together with this introductory paper, constitute the geologic-methods section. Together, these papers provide a comprehensive, up-to-date summary of the most useful methods currently employed in the search for stratigraphic traps. The section begins with a paper on “Strati-graphic-Trap Classification” by Gordon Rittenhouse. In this classification, he divides stratigraphic traps into (1) traps adjacent to unconformities and (2) traps not adjacent to unconformities. The former traps are separated further according to their position either above or below unconformities, whereas the traps not adjacent to unconformities are separated into primary (depositional) and secondary (diagenetic) types. Included within the latter category are fracture-porosity traps which are facies-related but are not the result of local structural deformation. Rittenhouse subdivides these four major classes of stratigraphic traps further and gives terminology to identify the different types.

Abstract A trap for hydrocarbons requires the simultaneous existence of (a) a reservoir, (b) an isolated region of low potential in the reservoir, and (c) a barrier (or seal) with high enough entry pressure to retain a commercially producible volume of hydrocarbons. Three kinds of traps exist—structural, stratigraphic, and hydrodynamic. All three kinds have a reservoir bounded by a barrier but differ in what causes the isolated area of low potential. In classification of hydrocarbon accumulations, the conditions that determined the present location of the accumulation should be used where they can be ascertained. In the stratigraphic-trap classification suggested here, primary emphasis has been placed on usability— i.e. , will the groupings help in the search for new hydrocarbon accumulations, and is the suggested terminology simple and descriptive enough to be accepted? A classification using the time relations between barrier and reservoir was considered and rejected. The suggested classification starts with the simple concept that stratigraphic traps are adjacent to unconformities or they are not. For traps that are not adjacent to unconformities, the reservoir and barrier may be (I) primary (depositional, usually facies-related) or (II) wholly or in part secondary (diagenetic). Those traps in contact with unconformities may be (III) below the unconformity surface or (IV) above it, or (V) both below and above it. This approach uses some of Levorsen’s ideas and eliminates some inconsistencies in his classification. Subdivision of these four major classes (facies-change traps, diagenetic traps, traps below unconformities, and traps above unconformities) allows more precise description of the different types of traps.

Abstract The procedure suggested herein for finding stratigraphic traps depends upon the development of an understanding of the depositional and structural framework within which the traps were formed. Utilization of all available geological and geophysical data, coordination of various earth-science disciplines, and testing of the resulting hypothesis are required. The approach proceeds from data gathering, through data analysis and formulation of an exploration hypothesis, to testing of the hypothesis by drilling. The main element of the exploratory method is in data analysis, where the explorationist (1) determines basin structure and type, (2) establishes a time-stratigraphic framework, (3) locates unconformities, (4) interprets the environmental facies within the stratigraphic framework, (5) reconstructs the paleogeography, and (6) predicts where stratigraphic traps ought to be. The exploration hypothesis subsequently is tested by drilling, and the additional data are used to accept, modify, or reject the original interpretations.

Abstract Accumulations of oil and gas are closely related to unconformities in almost every oil region of the world. Except for classifications and early attempts to utilize them in regional correlations, the location, origin, and geometry of successive unconformities largely have been ignored. Unconformities occur in every tectonic and depositional environment. They are most common on the continental platform, where disconformities occur in close rhythmic succession. In the coastal areas between basin and platform, frequent warping has resulted in intersecting low-angle unconformities. Basins have the fewest unconformities, but angular unconformities may occur associated with midbasin uplift and diapirism. In the typical platform area, traps above disconformities are mainly in quartz sandstones and are long and narrow; carbonate rocks are the principal reservoirs beneath disconformities. Traps in the hinge area between the platform and the basin are associated with low-angle unconformities; they are commonly very large and generally have arenaceous reservoir rocks. In basins not later deformed by tectonism, most unconformity traps are on the upthrown side of growth faults and on and around midbasin ridges, where submarine erosion has produced local unconformities. Reservoir rocks are commonly thin and discontinuous, fine-grained turbidites and residual sandstones.

Abstract Primary stratigraphic traps in sandstone involve lateral termination of the reservoir as a direct or indirect result of factors related to the depositional environment. Red Wash, Coalinga East, Pembina, Mitsue, Bell Creek, Cut Bank, Burbank, and Bradford are among the very few giant oil accumulations found in such traps. As these traps rarely can be detected by surface measurements, other discovery methods are essential. The understanding of depositional process and environment is a promising approach. Primary stratigraphic traps in sandstone are present in many facies, including fluvial, deltaic, shallow marine, and deeper marine. The largest sizes and greatest number occur in shallow-marine and shoreline environments. Knowledge of sandstone models of all kinds may provide valuable clues in interpreting fragmentary well data in terms of size, shape, trend, and characteristics of the reservoirs being sought. The distribution of many sandstone bodies may be controlled in part by underlying, commonly inconspicuous, erosional surfaces. Reconstruction of the paleo-topography of the unconformity thus may commonly delineate prospective trends. The distribution of trap barriers may be controlled by environment. For example, discrete shoreline sandstone bodies replaced updip by lagoonal shales are better prospects than those replaced updip by sandy (“leaky”) deltaic deposits. Such sandstones are more likely to be related to interdeltaic rather than deltaic areas. Most progress will come from further development and refinement of depositional models. A greater understanding of shallow-marine sandstone bodies is especially needed. Moreover, as exploration emphasis shifts offshore, there will be a growing premium on ability to recognize depositional models in the absence of cores and outcrops.

Abstract It is commonly impossible to distinguish structure due to deposition and erosion from tectonic structure. Moreover, stratigraphic agents producing trap limits are usually dependent on tectonic influences. It is thus impractical to contrast stratigraphic versus structural traps with the intention of searching for one type and not searching for the other. This is most clear in the case of carbonate traps where tectonism controls development of both erosional and depositional structure and affects depositional and diagenetic facies distribution. In the Florida-Bahamas carbonate province, intraplatform straits and basins are sites of negative residual Bouguer gravity anomalies. A correctable refractor near the top of Lower Cretaceous rocks is depressed in these same areas. Thus, present topographic lows overlie structural lows in the platform’s foundation. Similar relations are indicated for the Tampico-Tuxpan and Scurry reef platforms and are markedly evident in the Central Basin platform and the Leduc-Rimbey trend. This relation is a potentially useful one, because the geophysical anomalies reflecting the structures which control the position of the platforms commonly exceed those stemming directly from carbonate masses. Depositional and, to a degree, diagenetic facies have consistent topographic settings in both recent and ancient platforms. Calcarenites predominate at the edge; calcilutites and evaporites are most common in the platform interiors. Bases of platform-edge slopes are typically sites of deposition of allochthonous shallow-water sands mixed with coarse rubble containing balls of pelagic mud. Elevated edges commonly are leached and dolomitized, and dolomite is present in many places within the platform-interior evaporites. Great quantities of hydrocarbons have been found in the leached and dolomitized platform edges, in porous and permeable platform-interior dolomites, in dolomitized conglomerates bordering bases of platform slopes, and in fractured reservoir rocks in adjacent basin facies. The requirements for an oil field are structure, reservoir, seal, and a commercial quantity of hydrocarbons. Geophysical tools are best suited to discern structure. Velocities and reflection character also provide some insight to lithologic variations. Outcrop and subsurface studies enable mapping of distribution of reservoirs and seals. Slabbed cores from ancient carbonate rocks reveal sedimentary structures identical with those observed in recent carbonate units. Thus, study of modern carbonate deposits is a valuable aid in interpretation of rocks, and slabbed cores are essential for a detailed understanding of carbonate depositional and diagenetic history. Temperature and hydrocarbon-generating history of source beds can be discerned from the nature of organic matter remaining after oil and gas are gone. Knowledge of this relation enhances the ability to predict types of hydrocarbons to be encountered in a given region. Prospects for testing must be chosen on the basis of areal extent of structure and the regional distributions of reservoirs, seals, and hydrocarbons. Finally, management and backers should be prepared to drill two or three evaluation wells following the completion of a successful wildcat in a carbonate reservoir.

Abstract In recent years three developments which have evolved more or less independently, when related, may be of value to the petroleum industry. First is the recognition, through normal oil field development, that fractures are significant to both reservoir capacity and performance. Second is the fact that controlled laboratory experiments have produced, in increasing quality and quantity, empirical data on rupture in sedimentary rocks. These data have been segregated to demonstrate the individual control on rupture of several important parameters: rock type, depth of burial, pore pressure, and temperature. The third development consists of the discovery of new methods to recognize, evaluate, use, and, in some cases, see fractures in the subsurface. This discussion of these three developments may help geologists and engineers to find new approaches to exploration and exploitation of fractured reservoirs. Reservoir and production engineers presently make the greatest use of fracture data, but geologists should find this information useful in exploration for oil and gas trapped in subsurface fractures. Except in the search for extensions to proved fracture reservoirs, there is in the literature a paucity of clear-cut examples of the use of fracture porosity data in advance of drilling. For this reason, several speculative exploration methods discussed herein implement mapping of fracture facies as well as stratigraphic facies.

Abstract Well logs are used qualitatively to make Stratigraphic correlations and interpretations of depositional environment. Formational fluid properties, porosity, and compositional attributes are computed quantitatively from many well logs for use in preparation of exploration maps. Dipmeter data are treated statistically and incorporated with other log data for use in making stratigraphic interpretations and predictions. Computers can be used effectively to process digitized well logs and associated data; plotters are used to display exploration maps automatically.

Abstract A trap is of no value unless it has oil or gas in it. Prospecting, therefore, should include efforts to determine if petroleum was generated by the enclosing rocks and if it was likely to have collected behind the barriers that constitute the trap. Observations can be made to see if the rocks and fluids contain traces of hydrocarbon which would suggest that they are source rocks. Oil seeps from breached traps around the margin of a basin commonly suggest that similar traps may contain oil downdip. The key to stratigraphically trapped oil is the presence of barriers to fluid flow. Such barriers can be located by discontinuities in the patterns of fluid pressures. In mountainous areas, meteoric water commonly has gained access to strata which have regional continuity of permeability. Abrupt changes in water composition in these areas indicate barriers where stratigraphic factors may have preserved the petroleum.