Douglas W. Jordan

Hierarchical Levels of Heterogeneity in a Mississippi River Meander Belt and Application to Reservoir Systems: Geologic Note (1)

Six hierarchical levels of sand heterogeneity that would be present in analogous reservoirs are recognized in a Mississippi River meander-belt system in southeastern Missouri: (1) meander belt, (2) meander scroll, (3) channel, point bar, and splay, (4) lobe sheet, (5) bedding unit, and (6) laminae. Heterogeneity level 1 is similar in size to an oil field and consists of (1) 9 million ac-ft (11.1 billion m{3}) of high-permeability channel sands in numerous meander scrolls and (2) 4 million ac-ft (4.9 billion m{3}) of low-permeability clay plugs (in numerous abandoned channels) that separate a field into isolated pools and reservoirs. Overbank flood-plain muds capping the meander-belt system are barriers to vertical migration of fluids. Heterogeneity level 2 is the size of pools within a field and has 1.2 million ac-ft (1.5 billion m{3}) of highly permeable, laterally accreted channel and point-bar sands, partly or wholly isolated from other sand bodies by low-permeability, abandoned-channel clay plugs. Abandoned mud-filled chutes impede lateral migration of fluids in the upper pa ts of the meander scroll. The third level of heterogeneity is composed of about 70,000 ac-ft (86.3 million m{3}) of individual, permeable channel, point-bar, and crevasse-splay sand bodies, with numerous thin sheets and lenses of low-permeability muds and silts (derived from chute fill and mud drapes) that impede vertical migration of fluids. Heterogeneity level 4 is the lobe sheet unit (reservoir pay zone about 450 ac-ft or 555,000 m{3} in volume) comprising channel, point-bar, and splay sand bodies. The fifth level of heterogeneity is the single bedding unit (about 10 ac-ft or 12,000 m{3}) at the scale of a reservoir flow unit or perforated interval. Permeable, cross-bedded sand bundles are separated by low-permeability, inclined and horizontal mud-silt layers and lenses present along bed-set boundaries. Heterogeneity level 6 is individual sand laminae (grain-flow lenses or grain-fall sheets) separated by textural variations (and hence permeability-porosity variations) and isolated mud-silt laminae. Permeabilities range from 90-160 d in sand; several millidarcys to 50 d in muddy sand, silt, and sandy silt beds from the levee, crevasse, and abandoned-channel fill; and less than 10 md in muddy silts and muds from abandoned-channel and chute-fill mud sheets and laminae. At all levels within a fluvial meander-belt system, these permeability heterogeneities are the principal controls on productivity throughout the life of a reservoir.

Outcrop gamma-ray logging to improve understanding of subsurface well log correlations

Abstract Reliable, reproducible gamma-ray logs of outcrops have been generated by two techniques which aim to improve the visualization of interwell-scale lateral continuity (and discontinuity) of strata and to demonstrate the reliability and potential pitfalls of subsurface wireline log correlations. One innovative technique was developed which uses a standard gamma-ray sonde run from a logging truck in order to log vertical cliff or quarry faces. A second technique employs a hand-held gamma-ray scintillometer to log more easily accessible outcrops. Examples are presented from the Jackfork Group (Pennsylvanian), Arkansas, USA, of outcrop gamma-ray logging of both laterally continuous and discontinuous turbidites in structurally simple and complex settings. Because the strata from which the logs were measured can be visualized and discussed at the outcrop, these examples can clearly illustrate the following aspects of wireline log correlation: (1) reliability can be greatly improved by understanding expected subsurface depositional geometries and lateral facies changes; (2) wireline log correlations in stratigraphically and structurally complex settings, such as many oil or gas fields, may not be reliable without sufficient coring and special logging in addition to well testing; (3) erroneous correlations can result from the common practice of inferring three-dimensional rock geometries from two-dimensional well log data. Additional examples demonstrate that correlations and interpretations of subsurface wireline logs in both oil/gas fields and in exploration areas can be improved by comparing the subsurface logs with outcrop gamma-ray logs of nearby analogous strata.

Gamma-ray logging of outcrops by a truck-mounted sonde

An Innovative Technique Has Been Developed to acquire gamma-ray logs from rock outcrops. A standard logging truck is driven to the top of a quarry or cliff, and a gamma-ray sonde is lowered down the face by cable to the base of the cliff. The sonde is then raised at a constant rate, and gamma-ray measurements are continuously recorded, a method similar to logging a borehole. The logging rate is slower than conventional (borehole) rates since, on a cliff face, the gamma-ray tool is exposed to less than half of the rock mass normally encountered in a borehole. Outcrop logging of strata where the geometry and stratification character of the deposit is observed demonstrates the reliability and potential pitfalls in subsurface wireline log correlations and provides better visualization of interwell-scale lateral continuity (and discontinuity) of strata. These outcrop logs assist in developing an understanding of the properties of the strata responsible for a particular gamma-ray log response and demonstrate the necessary caution in interpreting vertical sequences, depositional environments, lateral continuity, and reservoir quality from gamma-ray logs.

Sedimentology and Facies of a Mississippi River Meander Belt: A Fluvial Model Based on a Significant Fluvial System: ABSTRACT

The meander belt of the Mississippi River in Southeastern Missouri, consists of four facies: river channel, chute, levee, and abandoned channel fill. A depositional model and vertical sequences have been developed from drill cores, vibracores, fathometer surveys, and mapping of these facies. This model and the vertical sequences compare very well to ancient sequences. The vertical sequences observed in cores through the various facies systems have systematic variations and associations that serve as models for meander belt fluvial systems.