Production Optimization of...Deltaic Reservoirs ... ...LL-652 Area, Lagunillas Field, Lake Maracaibo, Venezuela

RI0226. Production Optimization of Tide-Dominated Deltaic Reservoirs of the Lower Misoa Formation (Lower Eocene) LL-652 Area, Lagunillas Field, Lake Maracaibo, Venezuela, by W. A. Ambrose, E. R. Ferrer, S. P. Dutton, F. P. Wang, A. Padron, W. Carrasquel, J. S. Yeh, and Noel Tyler. 46 p., 29 figs., 1995. Print.


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ABSTRACT
Structurally complex, heterogeneous, tide-dominated deltaic reservoirs in the Lower Misoa Formation (lower Eocene C members) in the LL-652 area of Lagunillas field in the Maracaibo Basin, Venezuela, have produced 166 million stock-tank barrels (MMSTB) of oil but have a low recovery efficiency of 22 percent. These reservoirs will contain more than 900 MMSTB of unrecovered mobile oil when primary recovery operations at the current 80-acre well spacing end. In this study we characterized lower Eocene reservoirs in the LL-652 area, Lake Maracaibo, to improve estimates of hydrocarbon reserves, to identify potential areas for secondary-recovery projects, and to establish a field-depletion plan to evaluate advanced recovery opportunities and extended development.

A significant remaining oil resource lies in poorly drained or undrained reservoir compartments confined by a combination of complex structure and depositional heterogeneity. Sealing and partly sealing faults, including northwest-trending normal faults and younger north-northeast-trending reverse faults, bound large-scale structural compartments. Stratigraphic heterogeneity is controlled by dip-elongate, distributary-channel, and tidal-ridge sandstones that commonly pinch out over distances of less than 2,000 ft (<610 m). Because these two facies compose most of the reservoir sandstones, they contain most of the remaining oil.

The main control on porosity and permeability distribution in the C members in the LL-652 area is depth. Volume of quartz cement in particular influences reservoir quality, and because volume of quartz cement increases significantly with depth, reservoir quality decreases with depth. Within each reservoir interval, however, depositional architecture controls porosity and permeability distribution. For example, significant permeability contrasts (as much as three orders of magnitude) exist locally between distributary-channel and tidal-flat, fluvial-estuarine channel and distal deltafront, and distributary-channel and delta-front facies, where clay clasts at the base of the distributary-channel facies may retard vertical fluid flow.

Cumulative production varies greatly in each reservoir interval as a result of differences in ages of producing wells, greater-than-average production in areas of repeat section and inferred increased fracture permeability in zones of reverse faults, and differences in net thickness of perforated intervals. The tide-dominated deltaic depositional fabric, however, controls net sandstone and net pay thickness of each reservoir interval and therefore must affect cumulative production.

We made maps of hydrocarbon pore volume (SoPhiH) and remaining oil on the basis of improved petrophysical characterization and production apportioning to specific reservoir horizons by permeability feet (kh). These maps indicate that most remaining oil lies in the poorly developed and structurally complicated north part of the field and where narrow (less than 2,000 ft [<610 m wide), high-SoPhiH belts are intersected by sealing and partly sealing reverse faults. The original oil-in-place resource base of the C members in the LL-652 area increased by 867 MMSTB (60 percent) to 2,318.2 MMSTB, mainly in the C-3-X and C-4-X members, by our identifying additional reservoir areas and improving quantification oi porosity and other petrophysical parameters. Extended development on the current 80-acre (1,968-ft [600-m) well pattern that has 97 new wells will increase reserves from 127 to 302 MMSTB. However, 116 MMSTB, in addition to the 302 MMSTB, can be produced from 102 geologically based infill wells strategically targeted to tap areas of high remaining oil saturation in narrow sandstone bodies that pinch out over distances of less than the current well spacing. Horizontal and inclined wells in steeply dipping strata in the C-3-X and C-4-X members can capture additional volumes of poorly contacted mobile oil.

An integrated reservoir-characterization program that includes structural, stratigraphic, petrophysical, petrographic, production-engineering, and volumetric analyses can be used to improve oil-recovery operations in other mature, petroleum-producing provinces of Venezuela where many fields are nearing the first stages of primary depletion. The LL-652 area can serve as a model to demonstrate the most efficient means for recovering the remaining oil resource--a strategy targeting reservoir compartments defined by depositional-facies geometry and structure.

Keywords: Lagunillas field, Lake Maracaibo, lower Eocene, Lower Misoa Formation, reservoir characterization, resource-targeted reservoir development, tide-dominated deltaic reservoirs, Venezuela

CONTENTS
Abstract

Introduction

Objective and Study Area

     Data Base and Methods

     Geologic Setting

         Stratigraphic Setting

               Maracaibo Basin

               LL-652 Area

     Structural Setting

               Maracaibo Basin

               LL-652 Area

Depositional Systems

     Tide-Dominated Delta Model

        Distributary Channel

         Delta Front

         Tidal Channel and Tidal Flat

         Transgressive Sand Shoal and Shelf

         Fluvial-Estuarine Channel

     Three-Dimensional Facies Architecture

Permeability Trends and Diagenetic Controls on Reservoir Quality

     Reservoir Diagenesis

         Framework Mineralogy

         Diagenetic Controls on Reservoir Quality

Petrophysical Evaluation

     Model for Calculating Shale Volume (VSh)

     Porosity Model (Vsh-Porosity Transforms)

     Permeability Model (Permeability-Porosity Transforms)

     Water-Saturation Model

         Cementation and Saturation Exponents

     Fieldwide Formation Evaluation and Validation by Mapping

     Net-Pay Determinations

Sandstone Architecture and Recovery Optimization of Reservoir Intervals

     Depositional Controls on Net Pay and SoPhiH

     Reservoir Examples

         Upper C-4-X Submember

         Sandstone-Body Geometry, Lithology, and Depositional Systems

         Porosity and SoPhiH

       Net Pay, OOIP, and Remaining Oil

       Reservoir Development

       Middle C-3-X Submember

         Sandstone-Body Geometry, Lithology, and Depositional Systems

         Porosity and SoPhiH

         Net Pay

         OOlP and Remaining Oil

         Reservoir Development Opportunities and Strategies for Recovery Optimization and Reserve Growth

     New Extended-Development Locations

     Resource-Targeted lnfill Wells

     Horizontal and Inclined Wells

     Secondary Recovery

Conclusions

Acknowledgments

References

Figures

1. Average recovery efficiency of oil reservoirs versus depositional systems

2. Location of the LL-652 area and Lagunillas field in the Maracaibo Basin and major Eocene stratigraphic sequences in the Maracaibo Basin
3. Type log, LL-652 area

4. Location of wells, type well, and cored wells, LL-652 area

5. Paleotectonic maps showing chronology of Tertiary tectonic episodes in the Maracaibo Basin

6. Present structural styles in the Maracaibo Basin

7. Fault-bounded compartments in lower Eocene strata, LL-652 area

8. Tide-dominated deltaic depositional model of lower Eocene reservoirs, LL-652 area

9. Core description of distributary-channel and related facies and net-sandstone map, middle part of the Middle C-5-X submember, LL-3075 well

10. Core description of distributary-channel facies truncating delta-front facies, Middle C-3-X submember, LL-3074 well

11. Core description of medial delta-front facies, ripple cross-stratified and laminated sandstones, and symmetrical ripples at the crest of the tidal sand ridge overlain by laminated siltstone, Middle C-5-X submember, LL-3075 well

12. Core description of distal delta-front facies, Lower C-3-X submember, overlying shelf and transgressive sand-shoal facies, Upper C-4-X submember, LL-3074 well
13. Core description of progradational tidal-flat facies and photograph of mud-draped flaser ripples in high-tidal-flat facies, Middle C-4-X submember, LL-2850 well

14. Core description of fluvial-estuarine-channel, upper estuarine-channel-fill, and transgressive sand-shoal facies, Lower C-6-X submember, LL-3080 well

15. Typical percent-sandstone patterns in offlapping, progradational sequences in the tide-dominated deltaic facies tract, C members, LL-652 area, showing superposition and basinward progradation of distal delta-front, proximal delta-front, distributary-channel and channel-mouth-bar, overlain by lower delta-plain facies

16. Average compositional classification of lower Eocene C-member sandstones by facies and by member, LL-652 area

17. Porosimeter-measured porosity as a function of present burial depth and quartz cement volume, of C-3-X through C-6-X sandstones, as well as plot of log permeability versus porosity

18. Comparison of petrophysical properties between core data and those calculated using porosity values derived from Vsh-porosity transforms, C-3-X member, LL-2850 well, and in the C-6-X member in the LL-3080 well

19. Comparison of water-saturation values calculated by Indonesian, Simandoux, and Waxman-Smit models and comparison of Vsh, porosity and permeability derived from gamma-ray and resistivity logs

20. Southwest-northeast stratigraphic dip cross section A-A' from cores, Upper C-4-X submember, showing permeability variation and basinward thickening of subunit nos. 1 through 4 in the transition from proximal to distal delta-front facies

21. Net sandstone, Upper C-4-X submember, LL-652 area

22. Maps of reservoir volumetrics, Upper C-4-X submember, LL-652 area, including arithmetic average porosity, SoPhiH, net pay, OOIP, remaining oil, and cumulative oil production

23. Net-sandstone-thickness map, Middle C-3-X submember

24. Southwest-northeast stratigraphic dip cross section B-B' and northwest-southeast stratigraphic strike cross section C-C', Middle C-3-X submember

25. Maps of reservoir volumetrics, Middle C-3-X submember, LL-652 area, including arithmetic average porosity, SoPhiH, net pay, OOIP, remaining oil, and cumulative oil production

26. Reservoir volumetrics, C members, LL-652 area

27. Field extensions in the Middle C-3-X submember

28. Map of remaining oil, Middle C-3-X submember, showing proposed new well locations, recompletions, redrilled wells, infill wells, and horizontal and inclined wells for the C-3-X and C-4-X members

29. Net-sandstone map, Upper C-4-X submember, east part of field, showing pilot waterflood area


Citation
Ambrose, W. A., Ferrer, E. R., Dutton, S. P., and others, 1995, Production Optimization of Tide-Dominated Deltaic Reservoirs of the Lower Misoa Formation (Lower Eocene) LL-652 Area, Lagunillas Field, Lake Maracaibo, Venezuela: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 226, 46 p.