Applied Geology for the Petroleum Engineer

Based on a workshop co- sponsored by PTTC's South Midcontinent Region and the Oklahoma Geological Survey, August 20, 2003, Norman, Oklahoma.

BOTTOM LINE

Compartmentalization within sandstone reservoirs is chiefly related to several scales of geological complexity. The most important scales include those determined by the depositional setting and the resultant facies architecture, mud content and shale distribution, and grain size trends. Porosity and permeability are often correlated with grain size in sandstone reservoirs. Many of the geological complexities in sandstone reservoirs are smaller than seismic scale ('sub-seismic') and must be defined by log, outcrop, and/or core-based studies. Integrated geological-engineering models for the reservoir can be optimized when the causes of compartmentalization are understood.

PROBLEM ADDRESSED

This short course presented applied geological principals for petroleum engineers. Emphasis was on characterization of sandstone reservoirs, compartmentalization, and its effects on reservoir performance. In order for petroleum engineers to maximize production and to optimize reservoir management of sandstone reservoirs, it is important to understand reservoir architecture and the geological causes of compartmentalization in fluvial, eolian, shoreface, barrier island, deltaic and deepwater reservoir settings. Features critical to reservoir development include several scales of geological properties including depositional setting, facies stacking patterns, lateral and vertical variations in lithology and grain size, sandstone continuity and depositional architecture, effect of bounding surfaces and petrophysical properties. Many of these aspects are beneath seismic resolution or detection. For this reason, detailed outcrop, log and core-based geological studies can provide important constraints that should be incorporated into reservoir models by petroleum engineers.  

KEY WORDS:

Bounding Surfaces, Compartmentalization, Facies Architecture, Reservoir Heterogeneity, Sandstone Reservoirs, Sequence Stratigraphy, Formation

SPEAKERS

Dr. Roger Slatt, School of Geology and Geophysics, University of Oklahoma

TECHNOLOGY OVERVIEW

Reservoir heterogeneity, commonly below seismic resolution, is what makes the reservoirs more complex than conceptual models. Reservoir description and characterization requires multidisciplinary teamwork in order to assimilate the critical data.

Geological features of sandstone reservoirs that control reservoir performance include bed dimensions (size, geometry, architecture), structural attributes, grain size and composition, burial depth and history, and drive mechanism. Tools for reservoir characterization include conventional logs, conceptual model, seismic reflection, cores and borehole image logs, computer 3-D geologic models. Scales investigated by these techniques range from 10-3 to 10-6 feet.

From largest to smallest, the scales of geological heterogeneity that are important for reservoir characterization and development include:
• Fundamental lithology, e.g. clastic/sandstone, carbonates, or fractured shale.
• Fundamental depositional setting, e.g. continental, mixed or marine deposits.
• Depositional system, e.g. fluvial, eolian, lacustrine, alluvial fan.
• Subtype of depositional environment, e.g. meandering or braided fluvial deposit as opposed to a cuspate or tide-dominated delta.
• Further refinement of the depositional facies, e.g. fluvial channel versus overbank (floodplain), mud plug or point bar.
• Reservoir quality: Porosity and Permeability.
• Sub-seismic structural features such as faults, folds, diapirs, and fractures.

Fluvial Reservoirs
Remaining oil saturation after waterflood within a fluvial sandstone body often varies with grain size, and thus with permeability. But this relationship must be established for each sandstone succession and each channel.

Meandering River Facies Model: The coarsest lower portion of the sequence contains the least amount of mud. Multiple cross-cutting channel belts with highly elongate patterns (for individual channels) create complex heterogeneity. Trends are not laterally extensive.

Braided River Facies Model: Braided rivers tend to form on a steeper paleoslope. Their deposits can be divided into proximal, mid- and distal settings; in general becomes finer-grained downdip. Porosity and permeability tend to vary with grain size and depositional environment. For this reason, reservoir compartments tend to be laterally extensive and good results can often be expected from horizontal sidetrack completions within the pay zone.

An accumulation of river deposits can change from the braided to meandering type. For example, the Bartlesville Sandstone of NE Oklahoma consists of braided river deposits deposited formed during a lowstand systems tract and overlying meandering river deposits deposited as a transgressive systems tract. This change in depositional style should therefore be associated with a predictable change in reservoir performance. In this case, the meandering river sandstones are laterally discontinuous with interlayered mudstone, and are highly compartmentalized. The braided river sandstone is, in contrast, laterally continuous without much mudstone and is not highly compartmentalized.

Incised Valley Fill Reservoirs: Valleys are typically incised during falling stages of sea level. They are in turn filled during turn-around and rise in sea level. An idealized incised valley fill consists of incision during falling stage, fluvial deposits deposited during turn-around and early rise, and estuarine deposits deposited later during rising sea level. Once the valley has been filled it is capped by marine muds. Valley fills are typically encased in marine shale and so they are good stratigraphic traps. An example of this type of deposition is the "Stateline Trend" sandstones in Southwest Stockholm Field, Kansas. The wide range in environments within the valley fills can lead to highly divergent static pressure test results from components of this system.

Eolian Reservoirs
The most characteristic eolian (wind-deposited) deposits originate in inland sand seas and as coastal dune fields. Migrating dunes leave characteristic laterally-extensive bedsets with concave-up or parallel foreset cross bedding. Strike-oriented exposures of dune sediments show cross bedding that is similar to, but usually much larger-scale than trough cross bedding produced in fluvial settings. Examples include the Weber Sandstone, Rangely Field, Colorado and the Rotliegendes Sandstone from Pickerill Field, North Sea.

Several orders of bounding surfaces in the Tensleep Sandstone, Bighorn Basin, Wyoming indicate that permeability is significantly less across the more important bounding surfaces (first and second order) than across the third order bounding surfaces.

Structural Compartmentalization
Faults often compartmentalize the reservoir and result in different depth vs. pressure trends on opposite sides of the fault zone. Faults responsible for compartmentalization of reservoirs can be far below the scale necessary to recognize with seismic.

The Shale Gouge Ratio (SGR) is defined as:

SGR= Sum (Zone Thickness0 X (Zone Clay Fraction) x 100
                             Fault Throw

In different areas, a shale gouge ratio has been established which is distinctive for that area, and forms the basis for determining if a fault is apt to be sealing or not.. When sufficient gouge or filling-cement is present, it is likely that each fault is a seal, so that each fault block is a reservoir compartment. Other hints at fault compartmentalization include divergent pressure data, gas and oil production from different wells at the same structural elevation, or distinct groups of normalized Gas/Oil Ratios (GOR).

Shoreface Reservoirs
The shoreface is that zone downdip of the beach where wave energy impacts on the bottom causing ripples, trough cross beds and planar-tabular sedimentary structures in the shallow marine environment. Being "attached" to the beach at its updip margin, it is ideally located for sequence stratigraphic analysis. The grain size tends to coarsen-upward in the shoreface and has an upward-decrease in shale content. Typically the sediments of a shoreface parasequence exhibit upward-coarsening/thickening bedding and a sharp, often truncated top.

Shoreface sequences are internally complex. Individual sandstones develop during periods of relatively constant base level when sediment supply exceeds accommodation and thick progradational parasequences form. Overlying laterally continuous transgressive marine shales tend to vertically isolate the individual sandstones. Porosity-permeability values will vary with facies in shoreface sequences. High-resolution sequence stratigraphy should be applied to these sequences because of complex facies relationships that are formed during overall marine regression and transgression.

Barrier Island Reservoirs
Barrier island deposits consist of sandstone barriers that separate an updip lagoon-marsh-tidal flat complex from the open marine (shoreface) settings. Tidal channels that cut through the barrier are often associated with ebb and flood tidal deltas. Because tidal channel deposits are accumulated below base level they have the greatest opportunity for preservation in a transgressive system. More tidal channels are formed in mesotidal barrier islands than those deposited in microtidal conditions. During regression the entire facies tract may be preserved. An ideal vertical sequence consists of lower-middle shoreface muddy sandstone overlain by upper shoreface and beach fine-grained laminated sandstones, which in turn are overlain by dune sandstone. The top of the sequence is rarely preserved.

Facies relationships are complex within barrier island systems. Individual sandstone bedsets may be separated by lagoonal shales, which isolate the sandstones. In several barrier island reservoir studies, such as Bell Creek and Recluse fields, fluid flow rates, porosity and permeability have been shown to vary with facies and grain size.

Deltaic Reservoirs
Deltaic deposits almost certainly comprise the most geologically complex type of terrigenous reservoirs. Deltaic deposits form when sediment enters a standing body of water, such as the ocean, and many depositional facies are formed. Lateral and vertical facies relationships depend on the type of delta being constructed. Coarsening-upward vertical grain size trends are characteristic of most deltaic deposits, however it is important to understand which type of deltaic reservoir is present in order to predict facies architecture and maximize reservoir production

Fluvial processes dominate on lobate and elongate deltas where reservoir sands may develop in distributary channels, crevasse splays, and distributary mouth bars (river-dominated deltas) or channel and reworked delta-front sands (lobate deltas). Marine processes and longshore currents dominate on cuspate and tide-dominated deltas. Waves rework the delta front in cuspate deltas so that reservoirs are primarily developed in thick successions of strand and shoreface sandstones. Tidal energy dominates on tide-dominated deltas and reservoirs are developed within tidal current sand ridges or tidal channel sandstones.

Deepwater Reservoirs (Turbidites)
Sediments that are transported beyond the shelf margin into deeper water by sediment gravity-flow processes become potential reservoirs within the basin and continental slope settings. These types of discoveries have become increasingly more important in the last 10 years from the Gulf of Mexico, offshore W. Africa, Brazil, the North Sea, the SW shelf of Australia, and SE Asia.

Deepwater settings commonly contain three aerially-extensive potential reservoir elements: sediment sheets, channel fills, and levee/overbank deposits. Sediment sheets contain fine-grained turbidite deposits with repeated fining-up cycles caused by lateral shifting of channels and active sediment lobes. Shales located between the sandy portions of turbidite deposits may be laterally extensive or they may be truncated. The shales have different sealing potential depending on thickness and lateral extent. Sheet sandstones may be subdivided into different production zones by shaley intervals.

Just like fluvial sandstones, deepwater channel-fill reservoirs associated with turbidites can occur in braided or meandering geometries associated with levees. Channel sandstone complexes commonly occur with multiple scales of cross-cutting relationships. Such relationships can create very complex facies geometries (and hydrocarbon-water contacts) within deep-water channel systems.

Deepwater channel levee deposits are associated with crevasse splay silty sands and meandering type channels. Channel levee/overbank settings often show characteristic "gull wing" geometries on seismic sections, but channels and levee sandstone are not always in hydrodynamic communication. Sequence stratigraphic studies of outcrop analogs to deepwater channel/levee settings—such as occur in the Lewis Shale of Wyoming—are useful for reservoir modeling because critical relationships are often below seismic scale. Examples of deep-water sandstone reservoirs from the Midcontinent include the Jackfork Group.

CONNECTIONS:

Dr. Roger Slatt
University of Oklahoma
School of Geology & Geophysics
100 East Boyd Street, Suite 810
Norman, OK 73019
Phone: 405-325-3253
Email: rslatt@ou.edu
 

For information on PTTC’s South Midcontinent Region and its activities contact:

Charles Mankin, Director, Oklahoma Geological Survey
100 E. Boyd St., Room N131, Norman, OK 73019-0628
Phone 405-325-3031, Fax 405-325-7069, Email cjmankin@ou.edu

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