Stratigraphically complex fields

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Development Geology Reference Manual
Series Methods in Exploration
Part Geological methods
Chapter Evaluating stratigraphically complex fields
Author Diana Morton-Thompson, W. E. Galloway
Link Web page
Store AAPG Store

Stratigraphically complex fields are those that exhibit a high degree of vertical and lateral heterogeneity that is directly controlled by the environment of deposition (Table 1). This heterogeneity occurs at all scales (see Geological heterogeneities). It can result in a highly variable distribution of rock properties within one reservoir and the division of a single stratigraphic interval into multiple reservoirs that act as independent, self-contained compartments.

Degree of hetrogeneity

Table 1 Tentative, relative depositional heterogeneity ranking of reservoir facies based primarily on Texas petroleum fields
Depositional Environment Heterogeneity Rank
Siliciclastic Systems
  Submarine slope/fan/canyon fill, silty to muddy 9.0
  Braided streams 8.0
  Fine-grained meander belt 8.0
  Tidal flat and estuary 8.0
  Back barrier apron and tidal delta 8.0
  Fluvial or deltaic sandstone on carbonate shelf 8.0
  Alluvial fan 7.5
  Fluvial-dominated delta 7.5
  Coarse-grained meander belt 7.0
  Fan delta 7.0
  Shelf sandstones 7.0
  Submarine slope/fan/canyon fill, sandy 7.0
  Lacustrine shore zone and delta 6.0
  Barrier shoreface 6.0
  Eolian 4.5
  Wave-modified delta 4.5
  Wave-dominated delta 3.0
  Strandplain, sandy 2.5
  Barrier core 2.0
Carbonate Systems
  Evaporitic flats (sabkhas) 9.0
  Restricted platform, shoaling cycles, dolomitized 8.5
  Open shelves, extensive diagenesis 8.0
  Shelf edge reefs, drapes 8.0
  Shelf edge, slope or basinal 8.0
  Karstic unconformity zones 8.0
  Open shelves, platforms or ramps 7.0
  Small atolls, pinnacle reefs 7.0
  Oolitic bars and barriers 6.0
  Large atolls, pinnacle reefs 5.0

It is important to identify stratigraphic complexities early in field development so that (1) operations can be tailored to optimize recovery and (2) data necessary to properly characterize and manage the reservoir throughout its life can be obtained. It is also important to recognize stratigraphic complexities in mature fields because these fields offer the opportunity for significant improvements in production with relatively low risk.

Identification

Many criteria exist for recognizing stratigraphic complexities in new fields versus mature fields.

New fields

  • High degree of lithological variation seen in cuttings and core.
  • Small scale depositional packages
  • Vertical and lateral changes in wireline log character
  • Questionable correlation of depositional packages between wells
  • Presence of numerous vertical permeability barriers at a variety of scales
  • Complex porosity-permeability and capillary pressure relationships (a broad scatter in the data indicates different lithofacies and pore types)
  • Anomalous changes in pressure gradient
  • Variable oil and water chemical composition

Mature fields

  • Multiple fluid contacts at different structural elevations
  • Irregular reservoir distribution (such as a dry hole surrounded by producers)
  • Anomalous fluid relationships (such as a gas zone underlying an oil zone or an oil zone downdip of water)
  • Irregular and/or highly directional waterflood response
  • Poor injection profiles indicating irregular and poor waterflood sweep efficiency
  • Selective changes in produced oil and water chemical composition
  • Erratic pressures
  • Highly variable cumulative productions
  • Anomalous recovery factors
  • Variable GORgas-oil ratio (adjusted to standard operating conditions)
  • Difficulty in history matching reservoir simulation models with actual production

Problems

Problems associated with a stratigraphically complex field include difficulties in (1) drilling and completion and (2) reservoir characterization and management.

Drilling and completion problems

  • A variety of mud weights and types are needed to prevent formation damage in different producing zones.
  • Various stimulation treatments must be designed to optimize production for different stratigraphic zones or areas of the field.
  • Multiple completions and/or selective injection equipment are required.

Reservoir characterization and management problems

  • Correlations that subdivide the reservoir into meaningful producing zones must be consistent.
  • Maps must be generated that portray complex three-dimensional reservoir geometry.
  • The amount and type of pay in infill locations must be predicted.
  • Three-dimensional conductivity and “tracking” fluid flow must be understood.
  • Production or injection volumes need to be allocated to different zones or patterns.

Evaluation

Figure 1 Example of electrofacies map showing distribution of SP log patterns. (From Galloway and Cheng.[1])

Recognition and three-dimensional characterization of reservoir heterogeneity in a stratigraphically complex field requires a well-by-well analysis of the following:

  • Lithological information
  • Fluid types and saturations
  • Production history

Because of the inherent geological variability and the diverse developmental histories of different reservoirs, no one approach to reservoir characterization is universally applicable. Rather, evaluation must be tailored to integrate and maximize the use of all available geological, geophysical, geochemical, and engineering data.

The following steps outline a generalized plan for evaluating a stratigraphically complex field.

  • Understand the basic regional and local geological setting.
  • Describe lithofacies and interpreted depositional environment(s) (see Lithofacies and environmental analysis of clastic depositional systems).
  • Determine the relationship between lithofacies/depositional environment and reservoir distribution and quality.
  • Determine reservoir distribution.
  • Determine reservoir quality
  • Determine hydrocarbon occurrence
  • Integrate reservoir distribution, quality, and hydrocarbon maps with production histories and geochemical data.
  • Run reservoir simulation models to predict reservoir performance.
  • Use reservoir simulation model to manage field.
  • Refine reservoir description and calibrate reservoir simulation model.

See also

References

  1. Galloway, W. E., Cheng, E. S. S., 1985, Reservoir facies architecture in a microtidal barrier system—Frio Formation, Texas Gulf Coast: The University of Texas Bureau of Economic Geology, Report of Investigations, v. 144, 36 p.

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