Subsurface Mapping and Interpretation Tools Used in Exploration and Development of Petroleum Reservoirs Page1

NOTE! Please click on the figures and tables illustrated for enlarged view

SUMMARY

INTRODUCTION

EXPLORATION AND SEISMIC

The Space-Time Concept shown in Figure 1, uses four-dimensional frameworks designated by three spatial and one temporal coordinate (x y z) t. This must be considered before subsurface features are defined .¹

^{Figure 1: Space-Time Concept} 

In the early days little thought was given to subsurface work as the principal approach in exploration. The drill cuttings were thrown away and geologists only surface mapped. Around 1930 subsurface analysis became more important with the advent of mechanical well logging, geophysical, geochemical and statistical methods in discovering and developing petroleum reservoirs.¹

Petroleum geologists and engineers are using subsurface mapping successfully. This dynamic field always has new improvements coming on stream that use existing data from maps and logs, then correlates it into more precise and useful formats. The usefulness of subsurface geological mapping increases because as the amount of exploration rises, so does the difficulty in finding new structural features to drill; therefore more attention is paid to anomalies found by subsurface exploration methods.¹

Exploration employs velocity and neutron logging, density measurements, seismology, coring, magnetic and gravity reconnaissance as shown in Figure 2 to define sedimentary basin and underlying fault structures. Geochemistry tests subsurface formations. Well-cuttings are examined for possible petroleum staining.¹ Gamma ray and acoustic logs enhance 3-D seismic correlating the lithics evaluating well potential.¹¹

^{Figure 2: Magnetic and Gravity Maps}

Structural analysis evaluates craton style, unconformities, folding, multiple lithologies, facies changes and fault displacements. Faulting is evidenced by: structural discontinuity, repetition or omission of strata, brecciation, slickensiding, silicification, mineralization, log anomalies, change in core dip, Paleotologic irregularities, lost circulation, hole caving or deviation, sudden increase in drilling mud temperature and incomplete core recovery.¹ Large fields are found in structural traps, whereas smaller and more numerous fields are found in stratigraphic traps as described in Figure 3.¹

^{Figure 3: Types of Petroleum Reservoir Traps}

Seismic method measures elastic longitudinal and transverse elastic wave energy. Comparative velocities identify lithologies, gas, oil and water filled zones.³ Data is acquired over areas that do not have any well data by orientating seismic lines parallel to the target structure with adequate cross-line coverage density to yield optimum image quality as shown by Figure 4.¹¹

V_L = Velocity, K = modulus of compressibility, m = modulus of rigidity, d = density

Longitudinal equation V_L = Ö (3K + 4m ) /3d

Transverse equation V_T = Ö m /d

Figure 4: Seismic Lines across South Elk Basin

4-D seismic can predict movements of reservoir fluids between wells, assist in locating bypassed oil, avoid premature breakthrough, optimize infill well locations and evaluate pilot projects prior to full-scale implementation. The negatives are cost, not a stand alone technology and is geology depenedent.⁸

Seismic has evolved into a direct hydrocarbon indicator using high amplitude reflections, polarity reversal near gas, reflection time sag below gas causing shadows.¹

Drilling

Targeted infill drilling reduces development costs by almost 50% vs. blanket drilling plus adds more oil recovery in select field sections. This defines reservoir and non-reservoir rocks, rock-log correlations, cross flow barriers and facies for interpreting depositional and diagenetic history with an example shown in Figure 5.10 Core analysis reports include information about permeability, porosity, residual oil and total water saturation¹

Figure 5: Depositional Facies Map

Sidewall coring determines which zones should be production tested. Problems arise when sample recovery is usually less than 100% making correlation with correct section difficult.¹

Information about the type of formation hosting a petroleum reservoir can be gained by performing a drill stem test using pressure vs. time measurements.¹

The interpretation process determines which evaluation tools are used based on project economics and facies packages.²

Chevron Method calculates geologic risk by:

[Pgeologic = (Psource x Preservoir x Ptrap x Pdynamics)]

If any factor is zero, success is zero with drawbacks being time and cost. See Table 1 for checklist.⁷

Table 1: Risk Assessment Checklist

Rock typing measures lithology properties, porosity vs. permeability, resistivity vs. water saturation, apparent density, capillary-pressure curves, and residual hydrocarbon saturation vs. initial hydrocarbon saturation.²

Clastic percentage and ratios determine where the best reservoir potential is located in basins by categorizing pinching or thickening pay zones and indicating seal direction.¹

Clastic ratio equals (sandstones + shales)/(limestones + anhydrites) determine contour spacing and steepening of lithologies.

Clastic percentage equals (sandstone + shale)/(sandstone + shale + limestones + anhydrites).¹

 

Geophysical  WIRELINE LOGGING

Logging

Logging records various physical, chemical and electrical properties of rock/fluid mixtures in the wellbore enabling identification of hydrocarbon trapping conditions.¹

Porosity-lithology logs detect channels behind casings, casing leaks, packer leaks, tubing leaks, water influx problems, cross-flow from reservoirs and production problems such as 'thief' zones in water injection wells. Temperature logs indicate the top of casing cement and locations of gas in the wellbore determined from abnormally cool temperatures of entry point. Acoustic logs use sound waves in porosity and velocity calculations, plus predict blowout problems or detect unconsolidated formations. Caliper logs detect mud cake buildups, washouts, casing damage and measures hole volume. Dipmeter logs measures fault growth patterns and angles.¹

Resistivity logs such as: SP voltage logs analyze formation water resistivity in permeable zones. Induction logs induce currents into the formation via electromagnetic radiation coils, then measure invaded zone resistivity and water saturation. Micrologs use mini electrical arrays that calculate moved hydrocarbons in thin-bed resolutions. Laterolog uses multi electrode arrays in salty drilling muds for water saturation and porosity measurements.⁴

Complications of the measurements are contamination by cement, drilling mud clays, lube oil, stylolitic surfaces, uphole cavings, recirculated cuttings, facies sequence gaps and statistical fluctuations.¹

Advanced logging tools

High-tech logging can improve oil and gas production using multi-sonde logging tools for obtaining multiple data measurements per trip.⁴

Litho-density logs use photoelectric capture cross-sections (PE Curves) in identifying lithologies. Pulsed neutron logs bombard formations with neutrons then measure gamma radiation from neutron-hydrogen collisions in determining porosity and stratigraphy.

Gamma ray logs resolve lithology problems by radioactivity measurements with marine shales generating more gamma rays than sandstone, limestone or dolomite. Gamma with density logs can estimate lithologies (see Figure 6).⁶

Figure 6: Gamma and Neutron Curves

Problems occur with interpretation errors and tool malfunctions.⁴

 

CORRELATION AND MAPPING

 

Correlation considerations

Time stratigraphic analysis is the most critical component in all basic correlation procedures. This is based upon lithology continuity, unconformities, biostratigraphy, radioactive dating, type and degree of structural development, well-log and 3-D seismic profiles.¹

3-D seismic surveys, subsurface mapping and detailed core-porosity-permeability analysis (see Figure 7) result in high success rates for finding producers.¹¹

Figure 7: Core Porosity and Permeability

Correlation sections define sand continuity within the reservoir using shales as marker horizons. Development decisions are based on well perforations that maximize drainage efficiency, connectivity with competitive reservoirs (see Figure 8), number of wells required and the amount of remaining recoverable reserves.⁵

^{Figure 8: Correlations with Isopach Maps}

Tying seismic data

Seismic sequence analysis ties faults and horizons together by following laterally continuous seismic events. This is an advantage of seismic data over well information by eliminating 3-D ambiguity by using spatial gridlines.⁵

Synthetic seismographs and vertical seismic profiles tie faults and horizons using sonic and density log data. This eliminates 3-D ambiguity by using spatial gridlines as illustrated in Figure 9. Unfortunately problems occur from low quality data, varied interpretations, unwarranted faith in the checkshot data, severe horizontal velocity gradients, high noise areas, phantoming and complex geology.⁵

Figure 9: Seismic Profile Map

Computer correlation methods

Dresser process calculates different open hole and five cased-hole programs simultaneously.

Schlumberger's Faciology calculations define depositional environments, make well-to-well corrections, correlates seismic datas; aids in pooled field studies, mapping, contouring, reservoir modeling and simulations.⁴

Sentient "intelligence" computer systems emulate successful correlation techniques in predicting completed zone performance.⁴

Mapping

"An infinite number of variables in the composition and formation of sediments can be mapped."¹

Mapping of petroliferous traps involves creating structural, isopach, isochore, isolith, lithic percentage and ratio, isoperm, isopressure, isosalinity, isogravity maps. Problems arise when the lithic sampling or mapping is not done properly.¹ "The concept of an area being alive or dead with respect to hydrocarbons can be followed up by mapping both surface and subsurface hydrocarbon showings identifying their sources."⁶ Directional surveys aid in interpretation during map construction.⁵

Isopach maps show variations in strata thickness. Isochore maps represent drilled formation thickness. Lithofacies maps show lithology changes within the formation. Isopach and isofacies maps present 3-D models of the pay. Planiplastic maps show original sediments thickness before being erosion and truncation. Paleogeologic maps illustrate relative thickness, formational thinning rates, dip and folding subsequent to truncation. Facies will conform to older units creating unconformities. These maps do NOT depict the complete geologic story.¹

Echo mapping at 3.5kHz has revealed mass-flow sedimentation processes occurring frequently. This consideration is important in evaluating petroleum fields. There are direct correlations linking this process with climatic changes creating predictable deposition patterns over long time periods using similar paleodrainage channels. Five types of echo character shown in Figure 10 are bedded, transparent, hyperbolic, rugged and continental shelf.⁹

Figure 10: Five types of Echo Character Illustrated 

Contouring is important because vast amounts of oil have been found associated with unconformities and regional marker horizons. Good contouring makes the difference between productive wells and dry holes, especially in faults or anticlines.¹

 

CHEMISTRY AND RESERVOIR ANALYSIS

 

Chemistry

Geochemistry can determine amounts of oil produced by TOC of shales, then trace fault conduits to traps via source-reservoir relationships. Also the geochemistry of source rocks has determined that specific intervals like those in Figure 11 are more consistent producers of oil or gas.⁶ Problems arise from intrusives, pyrite, undercompacted formations, borehole washouts, low porosity dense rocks and sample Another use is in evaluating diagenetic changes such as: compaction, recrystallization, cementation, leaching and replacement.¹

Figure 11: Geochemistry Log

Geochemistry logs as shown in appendix M record TOC, HI, OI, S₁ (free-hydrocarbons), S₂ (generation potential), S₂/S₃ (gas vs. oil), kerogen type, fluorescence, Tmax, depth, vitrinite reflectance along with age, formation name, lithology and temp of rocks.⁶

Reservoir analysis

The geothermal gradient is key for the formation of mature petroleum reservoirs.⁶ But be careful of condemning a section based upon maturity is premature as petroleum may have originated in deeper structures.¹²

A formation tester measures the formation pressure and obtains a fluid sample thus using a relationship between formation pressure and time to calculate pressure gradients⁴

Geothermal gradient = Q = l ¶ T/¶ Z

Q = heat flow in cal/sq. cm sec

l = Thermal conductivity of a substance in cal/cm sec ° C

¶ T/¶ Z = geothermal gradient in ° C/cm where Z is positive downward as illustrated in Figure 12.⁶

Figure 12: Geothermal Gradient

The LOM (level of maturity) of petroleum gases can be evaluated on the basis of ¹³C analysis. The LOM of C₁-C₄ using ¹³C analysis is wide in immature reservoirs and narrow in mature reservoirs. This creates source rock fingerprints. By correlating with traps clear understandings of geologic history are obtained.⁶

Gas logs use gas chromatographs in detecting hydrocarbons from higher mud gas levels that correlate with reservoirs and wet/dry gas conversion points. Wet gas indicates oil production, whereas dry gas means gas or heavy oil. Disadvantage: not as effective in oil-rich muds.⁶

Thin section analysis by a petrogeologist involves cutting core, then polishing and staining it to bring out variations in color, texture and grain size. This aids in reservoir studies by showing the attraction between rock and fluids along which flow occurs.¹

Biological considerations

Economic biostratigraphy is precise correlating and age dating by identifying and evaluating evolutionary processes of various plants, animals and microfossils.¹

Groundwater analysis evaluates distribution, configuration and types of aquifers, relation of aquifers to stratigraphy and structure, rate, volume and direction of flow, composition and contaminates, origin, recharge and withdrawal, local and regional hydrodynamic gradients and effects of water systems on oil and gas.¹

---

 

^BIBLIOGRAPHY

^Books

^{LeRoy, L. W., LeRoy, D. O. and Raese, J. W., 1977, Subsurface Geology: Fourth Ed., Colorado School of Mines, Golden, CO.}

^{Helander, Donald P., 1983, Fundamentals of Formation Evaluation: Oil and Gas Consultants International, Tulsa, OK.}

^{Lowell, J. D., 1985, Structural Styles in Petroleum Geology: Oil and Gas Consultants International, Tulsa, OK.}

^{Crain, E. R., 1986, The LOG ANALYSIS HANDBOOK: Vol. 1, PennWell Publishing Company, Tulsa, OK.}

^{Tearpock, D. J., and Bischke, R. E., 1991, APPLIED SUBSURFACE GEOLOGICAL MAPPING: Prentice-Hall, Englewood Cliffs, NJ.}

^{Hunt, J. M., 1995, PETROLEUM GEOCHEMISTRY and GEOLOGY: Second Ed., W. H. Freeman and Co., New York, NY.}

^{Periodicals and Journals}

^{Otis, R. M. and Schneidermann, N., 1997, Process for Evaluating Exploration Prospects: American Association of Petroleum Geologists Bulletin, V. 81, No. 7, P. 1087-1109.}

^{Popov, S., 1998, Two Emerging Technologies Enhance Reservoir Management: Hart's Petroleum Engineer International, Jan., P. 43-45.}

^{Gaullier, V. and Bellaiche, G., 1998, Near Bottom Sedimentation Processes Revealed by Echo-Character Mapping Studies, Northwestern Mediterranean Basin: American Association of Petroleum Geologists Bulletin, V. 82, No. 6, P. 1140-1155.}

^{Montgomery, S. L., 1998, Permian Clear Fork Group, North Robertson Unit: Integrated Reservoir Management and Characterization for Infill Drilling, Part I-Geologic Analysis: American Association for Petroleum Geologists Bulletin, V. 82, No. 10, P. 1797-1814.}

^{Montgomery, S. L. and Morrison, E., 1999, South Eubank Field, Haskell County, Kansas: A Case of Field Redevelopment Using Subsurface Mapping and 3-D Seismic Data: American Association of Petroleum Geologists Bulletin, V. 83, No. 3, P. 393-409.}

^Internet

^{Palacas, Dr. J., Organic Geochemistry in Future Energy Development: USGS Fact Sheet FS-025-97, http://130.11.54.143/factsheets/organicgeochem/organic.html}

---

 

Rod_of_Rock@yahoo.com