CHARACTERIZATION OF CARBONATES AIDS EXPLORATION

Based on a workshop sponsored by PTTC's Eastern Gulf Region on September 2, 1999, in Jackson, MS.

BOTTOM LINE

Applying reservoir characterization during exploration for Lower Cretaceous strata in the Eastern Gulf region has great potential to improve the rate of drilling success. Geologically characterizing Lower Cretaceous Gulf carbonates facilitates identifying priority areas, focuses attention to areas with a greater probability for success, and ultimately saves exploration dollars.

PROBLEM ADDRESSED

Exploration requires successfully predicting reservoir trends and connectivity, lithology, and petrophysical properties and sedimentary models for seismic interpretation. In the Lower Cretaceous strata of the Eastern Gulf, the keys to successful exploration are understanding the structural, stratigraphic, depositional, and diagenetic processes.

KEY WORDS:

Geological Characterization, Carbonate Depositional Models, Diagenesis, Seismic Velocity

SPEAKERS

Lower Cretaceous Carbonate Depositional Framework:
Robert Scott, Precision Stratigraphy Associates

Outer Continental Shelf-James Limestone
Andrew Petty, US Minerals Management Service

Onshore Lower Cretaceous Carbonate Reservoirs
Rick Turner, Barrow-Shaver Resources Co.

TECHNOLOGY OVERVIEW

The Comanchean carbonate platform encircling the modern Gulf of Mexico includes one of the largest and long-lived reef tracts in earth history. This Lower Cretaceous platform is one of many platforms that formed in the Cretaceous Tethyan Realm. Understanding and describing geological controls of reservoir formation and distribution in the northeastern Gulf of Mexico may help exploration efforts not only in the Gulf, but in many analogous areas of the world.

The business impact of understanding the structural, stratigraphic, depositional, and diagenetic trends of the Lower Cretaceous of the eastern Gulf can be great. These geological features are the key to predicting reservoir trends and connectivity, predicting lithology and petrophysical properties ahead of the drill, and defining sedimentary models for seismic interpretation.

Geological Setting. The US Gulf Coast Lower Cretaceous carbonate section has numerous intervals with potential reservoirs and seals. Many of the rocks were subaerially exposed and formed leached porosity. Early marine cementation did not normally destroy all the depositional porosity in many facies. Meteoric leaching affected many stratigraphic horizons and commonly created porosity. Deep burial generally destroys porosity, although exploration targets include early-formed porosity that is now deeply buried.

By the end of the Albian, the composition of reefs had changed from coral-algal-microbe-dominated to rudist-dominated types. Mixed coral and rudist assemblages occupied shelf margins facing open oceans, but rudists characterized ramp buildups fringing shallow, intra-platform basins. Recognition of these changes is important in hydrocarbon exploration, because it greatly influenced reservoir facies development and geometry.

Two general shelf buildup models apply to the Comanchean Shelf. Near the shore, ramp grainstones and patch reefs developed. Toward the basin, shelfmargin reefs and high-energy carbonate sands formed large tracts of facies with preserved initial porosity. Back-reef areas of washover fans have greater probability of porosity preservation than the reef crest and fore reef where syndepositional micrite occluded much of the pore space.

Stratigraphic and Seismic Models. Regional unconformities divide the Lower Cretaceous of the US Gulf Coast into 13 mappable units. Many contacts developed updip, but not downdip. These sequences represent transgressive-regressive depositional events in 1-3/ my duration. Each contains smaller-scale cycles that typically shoal upwards in response to changes in relative sea level and climate. Each unit thickens downdip to the shelf margin where it is thickest.

There are several rules of thumb for carbonate seismology techniques indicating that rock-intrinsic properties— such as porosity, pore type, composition and grain size— are closely related to velocity: 1. Shallow-water, coarse-grained carbonate lithologies tend to have higher velocities than deep-water, mudrich carbonates. 2. Mineralogy has little influence on the seismic velocity in carbonates because the velocity in calcite and dolomite are nearly the same. 3. Porous carbonates generally have lower velocities, but the relationship is not simple. 4. Worldwide observations indicate that an impedance contrast can form between layers with different porosity types. 5. Both density and velocity correlate with porosity in carbonates. 6. Fluid presence and types seem to have only a small effect on velocities in carbonates, but pore type and shape are important.

Seismic Velocity Related to Pore Types. The relationship between seismic velocity and porosity is complicated by pore geometry and mineralogy, and the fact that most rocks have more than one pore type. However, some generalizations related to pore type include: 1. Intercrystalline and Interparticle Porosity -These pore types have a characteristically high internal surface area. Both primary and shear velocities are low for a given porosity and are strongly dependent on effective or net overburden pressure. 2. Moldic and Intraparticle Porosity -Moldic (MO) porosity is secondary, while most intraparticle (WP) and intracrystalline porosity is primary. Velocities in rocks with MO or WP pores are relatively insensitive to pressure changes and are generally high for a given porosity, since these pores are hard to deform. 3. Vug and Channel Porosity -Vuggy pores have strong rock frameworks and low internal surfacearea-to-porosity ratios. Thus, primary and shear velocities are relatively high at a given porosity and are insensitive to pressure changes. Channel pores are usually easy to deform, consequently these rocks usually have relatively low velocities at a given porosity. 4. Fenestral Porosity -This typically secondary pore type has openings larger than the grain-supported interstices. Velocities behave similarly to those with intercrystalline pores.5. Fracture/ Breccia Porosity -Fracture porosity evolves to breccia type, with increasing dislocation between opposing fracture walls. A unique feature of a fracture is that both primary and shear velocities depend on the direction of wave propagation relative to the fracture's orientation. If fractures are abundant and randomly oriented in a rock, the rock will behave isotropically. Although fractures contribute little to total porosity, they can greatly decrease the velocity of the rock. In deeply buried rocks, most small fractures are probably closed and have little effect on P-wave velocities.

Lithology and Log Characteristics. There are also a few rules of thumb for interpreting well logs with carbonates: 1. Spontaneous Potential (SP) logs -Detect permeable beds and boundaries; they also aid in estimating the amount of shale. SP logs commonly respond to carbonate mudstone in the same way as in clay shale. A shift from shale baseline in carbonates identifies an increase in carbonate grain size. 2. Resistivity logs -Help indicate porosity, permeability, and fluid type. 3. Gamma Ray (GR) log -Indicates radioactive mineral zones within carbonates originating from mineralogical changes, including shales, organics, glauconite zones, pedogenic zones, and hardgrounds. 4. Sonic log -Measures matrix porosity, not vuggy or fracture porosity, making the calculated sonic velocity of vuggy or fractured carbonates too low. Vug/ fracture porosity can be calculated by subtracting sonic porosity from total porosity measured by a density or neutron log. The difference is called secondary porosity index and can be mapped in carbonates. 5. Neutron log -Measures the hydrogen ion concentration. If properly calibrated, it will measure actual porosity in limestone.

Depositional Cycles. Carbonate sediments are excellent reflectors of variations in sediment supply and mineralogy. Repetitive sedimentary cycles of lithology and biota in carbonates can be created by episodic processes (repeating at unpredictable time intervals) or by oscillating processes (which are both regular and predictable). Earth precession, obliquity, and eccentricity influence carbonate sedimentary cycles at frequencies of 19-24 thousand years (Ka), 40-41 Ka, and 95-128 Ka, 413 Ka, 2 million years (Ma), respectively. Frequencies of these processes have changed through geological time.

Climatic changes also alter carbonate production rates. During times of greater production, sedimentation rates increase and the net result is aggradation and progradation. Coring such an event reveals a shoaling up facies succession that often has evidence of subaerial exposure at the top. Depositional cycles are extremely useful in predicting the stratigraphic level at which porosity may be present in carbonate settings.

Diagenetic Models. The high-risk element in defining carbonate plays is developing and preserving carbonate porosity. Much of the problem stems from diagenesis, which is the set of physical and chemical changes that occur to carbonates after deposition and prior to metamorphism. Modern carbonate sediments comprise metastable mixtures of aragonite, high-Mg calcite and low-Mg calcite. A conversion to stable low-Mg calcite alters original porosity. Tracking this conversion is important to predicting porosity.

Cementation, compaction, dissolution, neomorphism, microbial micritization, and dolomitization processes operate in marine, meteoric, and burial diagenetic environments. The main variables controlling diagenetic alterations include composition/ mineralogy of the sediment, pore fluid chemistry and flow rates, burial/ uplift/ sea-level changes, influx of different pore fluids, and climate. Meteoric diagenesis, occurring within the saturated or unsaturated "freshwater" zone, is critical to developing the porosity encountered in carbonate reservoirs. Diagenesis at subaerial unconformities within the meteoric zone affect porosity formation by rearranging pore networks without increasing porosity or permeability. Shorter periods of exposure may be associated with greater porosity and may form pore systems resistant to compaction.

Once buried below depth of freshwater and marine, the phreatic influence on the subsurface diagenetic environment is significantly different. Each basin has a unique brine history and prediction is difficult, although cementation and pore occlusion predominate. Porosity tends to decrease with depth due to cementation and compaction.

Fractures are extremely important in the delivery of hydrocarbons in carbonates. Generally, fracture porosity is not related to original rock fabric. Earlyformed fractures tend not to be as effective as latestage fractures for fluid flow. Early-formed fractures in carbonates may be distinguished by their highly variable length. They are filled with sediment and early cement, and are formed by volume change of sediment or by gravity. Meteoric fractures fill with cement or residual soils, cave filling, or caliche. In contrast, late-stage, deep-burial fractures cross-cut other sets, have facing surfaces that seldom match in shape, and few are offset by other fractures. In addition, they are uncemented, oil stained, and oriented relative to tectonic stresses.

CONNECTIONS:

Robert W. Scott
Precision Stratigraphy Associates
RRBox 103-3
Cleveland, OK 74020
Phone: 918-243-7871 E-mail rwscott@ix.netcom.com

Andrew Petty
US Minerals Management Service
1201 Elmwood Park Blvd., MS 5124
New Orleans, LA 70123-2394
Phone: 1-800-200-GULF, Fax 504-736-2905 E-mail andrew. petty@mms.gov

Rick Turner
Barrow-Shaver Resources Co.
100 East Fergeson, Suite 712
Tyler, TX 75702
Phone: 903-593-5221 Fax 903593-1692 E-mail barrowshaver@tyler.net

For information on PTTC’s Eastern Gulf region and its activities contact:
Ernest A. Mancini, Professor of Geology University of Alabama
Box 870338, 202 Bevill Bldg., Tuscaloosa, AL 35487
Phone: 205-348-4319, Fax 205-348-0818, E-mail emancini@wgs.geo.ua.edu

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