ABSTRACT

The Mississippi River drains an area of 3,344,560 sq km and delivers large quantities of fine-grained sediment to the Gulf of Mexico. Within the lower part of the river system and in the delta plain, lateral migration of the channel and dispersal processes result in deposition of a wide variety of sand bodies that form potential hydrocarbon reservoirs. The major sand bodies include fluvial channel-fill, bay-fill, distributary channel-fill, distributary mouth bar, and delta-front environments.

A falling sea level or a continued period of delta outbuilding results in progradation of the lower part of the fluvial system well out onto the continental shelf. Lateral migration of the channel concentrates sand from the varied lithofacies of the upper delta plain and Shallow marine deposits. The resulting vertical sequence shows a generally fining-upward sequence with bedding decreasing in thickness upward. The lower part of the sand body normally displays large-scale cross bedding and alternating well sorted and poorly sorted beds. Toward the top of the sand body, zones of convoluted laminations alternate with small-scale cross-laminations and planar bedding. Sand body thicknesses vary from 7 to 10 m to a maximum of 100 m. Log response displays a blocky pattern with a well-defined erosional base. In a strike direction, sand body dimensions range from 15 km to less than 2 km; in a dip direction, the sand body is highly elongated ("shoestring"), but porosity and permeability are highly variable, resulting in multiple reservoir units within the overall sand body.

The distributary channel fill results from the abandonment and subsequent filling of channels within the lower delta. Channel filling is overwhelmingly of a fine-grained nature, but the lower part of the fill often contains coarse clastics that are of reservoir quality. These sand bodies vary in width from 1 km to less than a few hundred meters, and thicknesses are highly variable, ranging from 10 m to only a few meters. Log response is often blocky, displaying both a sharp base and a sharp top. Channel abandonment is a complex process and can be initiated by log jams, loss of gradient, catastrophic processes such as abnormal floods and hurricanes, or changes in upstream cross-sections. The sands contain an abundance of transported organic debris, and distorted and disrupted bedding is a common characteristic. Graded beds are exceptionally rare; laminations tend to show sharp bases and tops, and porosity and permeability vary from layer to layer. Although the sand bodies of the distributary channel fill are relatively thin and are restricted laterally, the process of abandonment is rapid, and it is not uncommon for such units to be stacked vertically within the overall deltaic sequence, forming multiple reservoirs within the overall delta-plain deposits.

Overbanking, a common characteristic of the Mississippi River, generally forms at angles to the main channel patterns. Overbanking patterns vary in scale from small overbank splays (a few square kilometers) to large bay fills covering in excess of 100 sq km. Overbanking forms initially as a break in the major distributary channel during flood stage, gradually increases in flow and depth of scour through successive floods, reaches a peak of discharge and deposition, wanes, and becomes inactive. As a result of sedimentary compaction and continued subsidence, marsh deterioration proceeds and the bay-fill deposits are inundated by marine waters; the marsh reverts to a bay environment, thus completing a sedimentary cycle. The mass of sediment resulting from this cyclic process ranges in thickness from only a few meters to 30 m and requires only a few years for the thinner units to more than 300 years for the thicker bay-fill units. The smaller overbanks or crevasse splays are commonly referred to as "levee deposits" and do not generally contain enough coarse clastics to form reservoir-quality sand bodies. The large bay fills, however, contain an abundance of coarse clastics and form excellent reservoir facies. The vertical sequence of a bay fill displays a coarsening-upward and thickening-upward trend. The base of the sequence commences with a highly bioturbated (and often highly calcareous) clay that contains shallow neritic microfauna. This unit grades upward into well-laminated clays and silts displaying thin normal and reverse graded silt units. Fine-scale rippling is often observed within the siltier units. A bedding thickness increase and coarser clastic beds generally begin abruptly near the base of the main sand body and often display well-developed cross-laminations containing an abundance of transported organic debris. Each sandy layer increases in thickness upward, and sorting and permeability increase upward. The major coarse detritus unit, commonly referred to as the bay-fill distributary mouth bar, consists primarily of sand with thin stringers of silt and occasional clay laminations. The silty units are normally well sorted and display high porosity and permeabilities. Often the top of the sand body is highly bioturbated and highly cemented by both calcium and iron carbonates. The bay-fill sand bodies commonly form a blanket that, as a unit, can be correlated over

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several hundreds of kilometers. Widths of 10 to 15 km and lengths of 20 to 25 km are typical of the bay fills in the modern delta. Thickness is highly dependent on the depth of the bay being filled and ranges from 30 m to only a few meters. Within a given bay-fill sequence, thicknesses of the main sand body vary considerably, thickening near the multitude of channel units and thinning appreciably away from the channel axis. As a result, log response is highly variable, depending on the proximity to channels. Within the overall delta-plain deposits, bay fills are constantly taking place during the progradation of the delta facies and, with continued subsidence and compaction, tend to stack vertically, forming multiple stacked reservoirs. It is the writers' belief that bay-fill reservoirs account for much of the production from shallow marine deposits within the Gulf Coast Tertiary sequences.

The major processes operative at a river mouth or terminus are significantly important in controlling the distribution and geometry of the subaqueous delta facies. Modern deltas tend to display three main distributary-channel patterns: single, rejoining, and bifurcating. Deltas displaying single channels are often characterized by bell-shaped river mouths and tend to form in settings of high to moderate tidal range, steep offshore slopes, and moderate to high wave climates. Such distributary patterns are characterized by highly elongate sand bodies that are dominated by sandy fills containing large-scale, multidirectional cross bedding. Rejoining channel patterns display complex bifurcating and rejoining (sometimes referred to as anastomosing pattern), and often only a few of the channels tend to take the bulk of the sediment and water discharge at any one time. Erratic flood discharges, relatively large bedload, high to moderate tidal ranges, and relatively steep offshore gradients characterize this type of distributary pattern. In the Mississippi River delta, the bifurcating channel pattern is most common. Channel complexity and number of distributary channels increase as delta progradation continues, thus forming a highly efficient sediment dispersal system. Low offshore slope, small tidal ranges, relatively high subsidence rates, and generally low to moderate wave climates tend to favor this type of distributary pattern. In all deltas prograding into a marine environment, density differences exist between the effluent waters and basinal waters, and thus buoyancy of the effluent sediment plume tends to be the controlling factor in determining the geometry of the resulting river-mouth deposits. Although the effluent plume contains an abundance of suspended sediments, in only a few river systems are suspended-sediment loads or temperature differences great enough to drastically influence the density of the effluent plume to form underflows. In most cases, buoyancy and channel gradient are the controlling factors in determining the distribution of deposition at a river mouth. Coastal currents, tidal influence, and wave climates often exert far greater control on the resulting geometry of the river-mouth depositional sequences.

As the turbid, freshwater effluent plume leaves the confines of the distributary-channel mouth, the plume spreads buoyantly over the denser marine basinal waters. As a result, velocity within the effluent plume tends to diminish in the seaward-spreading plume. The coarsest sediment carried by the river is thus deposited near the river mouth and the finer clastics are spread progressively farther seaward in an ever-widening and -decreasing velocity effluent plume. This process results in building the classical vertical sequence that is shown in virtually every geology textbook: a coarsening- and thickening-upward sequence. The finer grained sediments, referred to as prodelta deposits, contain a full marine microfaunal assemblage, are characterized primarily by parallel color laminations and thin silt stringers, and are extremely widespread. These deposits are often quite thick, in excess of 100 m, and are frequently overpressured. The marine prodelta deposits grade upward into the highly laminated and bedded silts, sands, and clays that form the delta-front sequence. Bedding within this unit gradually thickens upward, and coarser clastics become more abundant. Graded bedding, both normal and reverse, is extremely common, and rippled sandy units are exceptionally abundant at the top of the unit. Although sand content rarely exceeds 40 percent, the coarser units are extremely well sorted and display high porosities and permeabilities. Because these laminated units are the result of deposition by a constantly shifting buoyant plume, the thin sandy units often display high lateral continuity. Delta-front deposits can be correlated considerable distances as a unit and tend to thicken appreciably in the vicinity of major distributaries. The thickness of this predominantly sand/silt unit varies from 30 to 70 m in the modern delta. These thin-bedded deposits are extremely common in deltaic sequences and form excellent reservoir facies (often referred to as low-resistivity pay zones). The major sand unit associated with individual distributary channels is called the distributary-mouth bar. Cross bedding, scour and fill, contorted layers, and small-scale cross laminations are the most common types of stratification preserved in these deposits. Transported organic debris and concentrations of mica are often extremely common within the upper part of the distributary-bar deposits. Thicknesses of the coarser clastic unit are highly variable, ranging from in excess of 80 m to 10 m. The deposits are generally wedge shaped in a strike direction, with the thicker portion being found near the channel. The deposits thin laterally and generally extend for distances ranging from 4 km to less than 1 km. Even in basins displaying low littoral currents and low tides, the sand body is often displaced on the downdrift side of the channel. In a dip direction, the sand body can display considerable continuity and tends to vary in thickness along its length, often expanding in thickness rapidly. Even though this thick detrital unit has high lateral continuity, numerous small faults, thin clay drapes, and rapid changes in porosity often limit reservoir continuity. Frequently, calcareous cementation forms across the top of the sand body, a function of migrating pore fluids and early diagenic biochemical processes.

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These major sand bodies -- fluvial channels, distributary-channel fills, bay fills, delta-front, and distributary-mouth bar deposits -- are the only major reservoir facies commonly found in the regressive delta-plain deposits. Although all of these deposits represent potential reservoirs, bay-fill and thin-bedded delta-front deposits appear to be the most prolific hydrocarbon reservoirs in the Gulf Coast sedimentary sequences.

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