Daniel A. Busch

Genetic Units in Delta Prospecting

Deltas generally are formed at river mouths during stillstands of sea level under conditions of cyclic transgression or regression. Consequently, they are rarely isolated phenomena, but form in multiples in a predictable fashion. Reservoir facies consist of continuous and discontinuous, bifurcating channel sandstones and delta-front sheet sandstones. The channel sandstones generally thicken downward at the expense of the underlying prodelta clays and may replace selected parts of the delta-front sheet sandstones. The lithologic components of a deltaic complex are interrelated and are referred to collectively as one type of Genetic Increment of Strata (GIS). The GIS is a vertical sequence of strata in which each lithologic component is related genetically to all the others. It is defined at the top by a time-lithologic marker bed (such as a thin limestone or bentonite) and at the base by either a time-lithologic marker bed, an unconformity, or a facies change from marine to nonmarine beds. It generally consists of the total of all marginal marine sediments deposited during one stillstand stage of a shoreline, or it may be a wedge of sediments deposited during a series of cyclic subsidences or emergences. An isopach map of a GIS clearly shows the bifurcating trends of the individual distributari s and the shape of the delta, regardless of the variable lithology of the channel fills. A Genetic Sequence of Strata (GSS) consists of two or more contiguous GISs and, when isopached, clearly defines the shelf, hinge line, and less stable part of a depositional basin. An isopach map of the McAlester Formation of the Arkoma basin is a good example of a GSS. The oil-productive Booch sandstone is a good example of a deltaic complex occurring within a GIS of this GSS. The upper Tonkawa, Endicott, and Red Fork sandstones of the Anadarko basin are identified as deltaic accumulations within different GISs. A hypothetical model serves as a basis for establishing the criteria for (1) recognizing successive stillstand positions of a shoreline, (2) predicting paleodrainage courses, (3) predicting positions of a series of deltaic reservoirs, (4) locating isolated channel sandstone reservoirs, and (5) tracing related beach sandstone reservoirs.

Prospecting for Stratigraphic Traps

Many stratigraphic traps are related directly to their respective environments of deposition. An understanding of the depositional environment is essential to successful prospecting for oil or gas in this type of reservoir. Isopach studies of shale sequences directly above, or both above and below, a lenticular reservoir sandstone are of considerable value in reconstructing depositional environments. Variations in thickness of such shale intervals, either directly above a reservoir sandstone, or embracing it, are completely independent of present-day structural configuration. Isopach maps of such genetic sequences serve as realistic indicators for locating certain lenticular sands. Depositional trends of beach sands, strike valley sands, and offshore bars are determined r adily from such studies. Structure maps, constructed on a reliable time marker within the arbitrarily selected genetic interval, serve as a means of locating oil or gas accumulation within any of these reservoir types. In all such studies electrical log data are essential since such genetic sequences seldom are named formational units. The thinner the genetic sequence the greater the necessity for accurate selection of correlation points on electric logs. Deltaic reservoirs are poorly understood and only rarely recognized by the geologist. This type of reservoir is, nevertheless, abundantly preserved in the sedimentary section. Regional isopach studies of depositional environment are a prerequisite for the construction of meaningful exploration maps of this type of reservoir. An understanding of the trends of distributary fingers and of the influence of differential compaction in producing drape structures, likewise, is essential.

Stratigraphic Trap Accumulation in Southwestern Kansas and Northwestern Oklahoma

An oil- and gas-bearing inlier of Morrowan sandstones presents a paradoxical situation in that water, oil, and gas are present in an inverted structural relation, some of the gas being structurally lower than oil and water. A map of the pre-Pennsylvanian topography reveals that a modified trellis drainage system was developed on subaerially exposed Mississippian strata. Southwestward-tilted resistant limestones of the Chesterian Series stood out as subparallel cuestas; intervening erosional valleys developed on interbedded nonresistant shales. Morrowan strata were deposited under conditions of cyclic marine southwest-to-northeast transgression, which was interrupted by several minor regressions. Sands were deposited during the regressive phases, and shales during transgressive phases. Small-scale structural noses and closures on the Inola limestone are largely the result of differential compaction of shales deposited over buried pre-Pennsylvanian topography. Detailed mapping affords a logical explanation for each producing well and all but a few dry holes in the Second Morrow sand of the Harper Ranch field of Clark County, Kansas. The anomalous distribution of some gas accumulation is explained by Gussows principle, modified to apply to a stratigraphic trap.

A.A.P.G. Continuing Education Program: ABSTRACT

The A.A.P.G., in recognition of the constant and rapid changes occurring in the art of oil exploration, is initiating a program of continuing education to affiliated and cooperating societies, oil companies, and university departments of geology. A series of 12-hour courses will be presented by the top authorities in this country on the following subjects: Stratigraphic Principles and Practice, Structural Geology, Petroleum, Economics, and Electronic Data Processing. It is anticipated that some local geological societies might wish to schedule lectures on only several course offerings, whereas others might wish ultimately to schedule the entire program over a period of years. Two lecture series per year are considered to be a realistic course offering for local geological societies, whereas only one per year might be all that a university geological department can justify. Oil companies might find it more expedient to sponsor three or four topics in concentrated fashion all within one week. The entire program is designed to update the technical background of practicing geologists. End_of_Article - Last_Page 1748------------

Comparative Study of Lenticular Reservoir Sands: ABSTRACT

Lenticular reservoir sands, when studied in the light of their depositional environment, shed considerable light on their nature and distribution. Lower Pennsylvanian sands of the Oklahoma part of the McAlester-Arkansas Valley Coal basin are lenticular in character and occur on the fringing shelf areas of the embayment. Successive epochs of Early Pennsylvanian subsidence are characterized by the deposition of lenticular sands, principally of the deltaic type. Examples of deltaic lenticular sands are known to occur in the Booch, Bartlesville, and Prue sandstone formations. Depositional environments are reconstructed by means of drawing isopachs of the genetic stratigraphic intervals in which these sands occur. Such maps serve to negate any significance of present structure and reveal the (1) shoreline trends, (2) distribution of shelf areas, and (3) the principal direction from which the sediments were derived. Additional maps of the individual reservoir sands, such as structure, thickness, isopotential, and reciprocal isopachs, reveal the nature of the lenticular sands within the genetic units. A knowledge of the origin and distribution of such sand bodies, both ancient and recent, is considered essential to the discovery of new stratigraphic traps of this type and to their exploitation once they are discovered. End_of_Article - Last_Page 429------------