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==Geologic architecture of the Arecuna field==

==Geologic architecture of the Arecuna field==

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Methods14ch08f01.jpg|{{figure number|1}}Location of the Arecuna field study area in the heavy-oil belt (modified from Erlich and Barrett, 1992).

Methods14ch08f01.jpg|{{figure number|1}}Location of the Arecuna field study area in the heavy-oil belt (modified from Erlich and Barrett<ref>Erlich, R. N., and S. F. Barrett, 1992, [http://archives.datapages.com/data/specpubs/basinar3/data/a136/a136/0001/0300/0341.htm Petroleum geology of the eastern Venezuela Foreland Basin], in R W. Macqueen and D. A. Lechie, eds., Foreland basins and fold belts: [http://store.aapg.org/detail.aspx?id=143 AAPG Memoir 55], p. 341-362.</ref>).

Methods14ch08f02.jpg|{{figure number|2}}Reference log of the main producing interval at Arecuna field. Depth is in feet.

Methods14ch08f02.jpg|{{figure number|2}}Reference log of the main producing interval at Arecuna field. Depth is in feet.

Methods14ch08f03.jpg|{{figure number|3}}Simplified stratigraphic cross section illustrating the flooding surface and lignite occurrence that defines the key stratigraphic subdivisions of Arecuna field.

Methods14ch08f03.jpg|{{figure number|3}}Simplified stratigraphic cross section illustrating the flooding surface and lignite occurrence that defines the key stratigraphic subdivisions of Arecuna field.

===Stratigraphic Framework===

===Stratigraphic Framework===

The main oil-bearing zones in the Arecuna field occur in the upper part of the Merecure Formation and throughout the Oficina Formation. The productive reservoir section is as much as 450 m (1500 ft) thick, and the stratigraphic analysis indicates that these productive intervals can be subdivided into 11 major stratigraphic units, three in the Merecure Formation and eight in the Oficina Formation ([[:File:Methods14ch08f02.jpg|Figure 2]]). The units were identified by systematic vertical changes in bedding architecture (readily apparent on well-log traces) occurring above successive, laterally persistent high gamma-ray (or minimum spontaneous potential [SP]) shale markers that are interpreted as surfaces of maximum marine or lacustrine flooding. Widespread lignite beds also were used to divide the stratigraphic section ([[:File:Methods14ch08f03.jpg|Figure 3]]). Because peat accumulation and preservation as lignite can occur only in the absence of significant clastic deposition (Hamilton and Tadros, 1994), lignite beds represent substantial periods of shutoff in sediment supply and approximate time lines separating successive depositional systems.

The main oil-bearing zones in the Arecuna field occur in the upper part of the Merecure Formation and throughout the Oficina Formation. The productive reservoir section is as much as 450 m (1500 ft) thick, and the stratigraphic analysis indicates that these productive intervals can be subdivided into 11 major stratigraphic units, three in the Merecure Formation and eight in the Oficina Formation ([[:File:Methods14ch08f02.jpg|Figure 2]]). The units were identified by systematic vertical changes in bedding architecture (readily apparent on well-log traces) occurring above successive, laterally persistent high gamma-ray (or minimum spontaneous potential [SP]) shale markers that are interpreted as surfaces of maximum marine or lacustrine flooding. Widespread lignite beds also were used to divide the stratigraphic section ([[:File:Methods14ch08f03.jpg|Figure 3]]). Because peat accumulation and preservation as lignite can occur only in the absence of significant clastic deposition,<ref>Hamilton, D. S., and N. Z. Tadros, 1994, [http://archives.datapages.com/data/bulletns/1994-96/data/pg/0078/0002/0250/0267.htm Utility of coal seams as genetic stratigraphic sequence boundaries in nonmarine basins: An example from the Gunnedah Basin, Australia]: AAPG Bulletin, v. 78, no. 2, p. 267-286.</ref> lignite beds represent substantial periods of shutoff in sediment supply and approximate time lines separating successive depositional systems.

Architecture of the stratigraphic framework defined from well control mirrors that of the 3-D seismic data ([[:File:Methods14ch08f03.jpg|Figure 3]]). Key stratigraphic surfaces readily identified on the well-log correlations (FS 40, 43, 45, 50, 60, 70, 90, and 100) were also readily identified on the seismic data ([[:File:Methods14ch08f04.jpg|Figure 4]]). Both on the well logs and on the seismic data set, these sequences were characterized by uniform thicknesses and conformable seismic surfaces (except where postdepositional faulting removed section). Stratigraphic subdivision from well control was difficult between FS 60 and 70, which is consistent with the downlapping and truncated seismic geometries evident on the 3-D data log in this stratigraphic interval.

Architecture of the stratigraphic framework defined from well control mirrors that of the 3-D seismic data ([[:File:Methods14ch08f03.jpg|Figure 3]]). Key stratigraphic surfaces readily identified on the well-log correlations (FS 40, 43, 45, 50, 60, 70, 90, and 100) were also readily identified on the seismic data ([[:File:Methods14ch08f04.jpg|Figure 4]]). Both on the well logs and on the seismic data set, these sequences were characterized by uniform thicknesses and conformable seismic surfaces (except where postdepositional faulting removed section). Stratigraphic subdivision from well control was difficult between FS 60 and 70, which is consistent with the downlapping and truncated seismic geometries evident on the 3-D data log in this stratigraphic interval.

===Depositional Systems===

===Depositional Systems===

Once the stratigraphic framework was established, gross sandstone and log-facies maps (Galloway and Hobday, 1983) were constructed for the major stratigraphic units, to interpret the depositional setting of the Arecuna reservoirs and to allow prediction of reservoir thickness and continuity along the proposed horizontal-well trajectories in the interwell areas. The Arecuna reservoirs are interpreted as deposits of bed-load and mixed-load fluvial systems. Depositional systems of genetic stratigraphic units Merecure A and Oficina A and B illustrate the range of reservoir styles at Arecuna field.

Once the stratigraphic framework was established, gross sandstone and log-facies maps<ref>Galloway, W. E., and D. K. Hobday, 1983, Terrigenous clastic depositional systems: Applications to petroleum, coal, and uranium exploration: New York, Springer-Verlag, 423 p.</ref> were constructed for the major stratigraphic units, to interpret the depositional setting of the Arecuna reservoirs and to allow prediction of reservoir thickness and continuity along the proposed horizontal-well trajectories in the interwell areas. The Arecuna reservoirs are interpreted as deposits of bed-load and mixed-load fluvial systems. Depositional systems of genetic stratigraphic units Merecure A and Oficina A and B illustrate the range of reservoir styles at Arecuna field.

====Merecure Unit A====

====Merecure Unit A====

Merecure Unit A, defined between flooding surfaces 90 and 100 and locally referred to as the U2 and U3 sandstone ([[:File:Methods14ch08f02.jpg|Figure 2]]), is interpreted as the deposit of a large-scale, braid-plain system. The unit varies from 33 to 51 m (110 to 170 ft) thick, and gross sandstone varies from 6 to 36 m (20 to 120 ft) thick. Although gross-sandstone trends are generally very broad, several digitate, north-oriented sandstone-rich axes are apparent ([[:File:Methods14ch08f05.jpg|Figure 5]]). Small pods where gross sandstone is thin occur only locally in the broad, sandstone-rich trends. These sandstone trends suggest weakly confined river flow across a broad alluvial plain. The digitate sandstone-rich trends define axial channel belts, and the local pods, where net sandstone thins, represent areas of lacustrine ponds that developed intermittently during coarse, clastic-sediment bypass. Facies analysis, based on gamma-ray and SP log patterns, indicates that the unit is dominated by aggradational, blocky log patterns, and these are attributed to amalgamated sandstone channel fills that accumulated in the axial channel complexes ([[:File:Methods14ch08f06.jpg|Figure 6]]). Interbedded mudstones displaying thin, spiky, and serrate gamma-ray and SP log motifs accumulated in the interaxial areas where intermittent coarse-clastic bypass led to local lacustrine inundation.

Merecure Unit A, defined between flooding surfaces 90 and 100 and locally referred to as the U2 and U3 sandstone ([[:File:Methods14ch08f02.jpg|Figure 2]]), is interpreted as the deposit of a large-scale, braid-plain system. The unit varies from 33 to 51 m (110 to 170 ft) thick, and gross sandstone varies from 6 to 36 m (20 to 120 ft) thick. Although gross-sandstone trends are generally very broad, several digitate, north-oriented sandstone-rich axes are apparent ([[:File:Methods14ch08f05.jpg|Figure 5]]). Small pods where gross sandstone is thin occur only locally in the broad, sandstone-rich trends. These sandstone trends suggest weakly confined river flow across a broad alluvial plain. The digitate sandstone-rich trends define axial channel belts, and the local pods, where net sandstone thins, represent areas of lacustrine ponds that developed intermittently during coarse, clastic-sediment bypass. Facies analysis, based on gamma-ray and SP log patterns, indicates that the unit is dominated by aggradational, blocky log patterns, and these are attributed to amalgamated sandstone channel fills that accumulated in the axial channel complexes ([[:File:Methods14ch08f06.jpg|Figure 6]]). Interbedded mudstones displaying thin, spiky, and serrate gamma-ray and SP log motifs accumulated in the interaxial areas where intermittent coarse-clastic bypass led to local lacustrine inundation.

The sandstone-dominant lithology, broad sandstone distribution ([[:File:Methods14ch08f05.jpg|Figure 5]]), and widespread blocky bedding architecture ([[:File:Methods14ch08f06.jpg|Figure 6]]) provide evidence of river flow that was weakly confined across a broad alluvial plain, although there are well-developed interaxial belts that are sandstone poor. Unconfined bed-load fluvial systems of the Canterbury Plain, New Zealand ([[:File:Methods14ch08f07.jpg|Figure 7]]; Leckie, 1994), perhaps provide the closest modern analog to Merecure Unit A, although these modern systems principally carry boulders, gravel, and very coarse sandstone.

The sandstone-dominant lithology, broad sandstone distribution ([[:File:Methods14ch08f05.jpg|Figure 5]]), and widespread blocky bedding architecture ([[:File:Methods14ch08f06.jpg|Figure 6]]) provide evidence of river flow that was weakly confined across a broad alluvial plain, although there are well-developed interaxial belts that are sandstone poor. Unconfined bed-load fluvial systems of the Canterbury Plain, New Zealand ([[:File:Methods14ch08f07.jpg|Figure 7]]),<ref>Leckie, D. A., 1994, [http://archives.datapages.com/data/bulletns/1994-96/data/pg/0078/0008/1200/1240.htm Canterbury Plains, New Zealand: Implications for sequence stratigraphic models]: AAPG Bulletin, v. 78, no. 8, p. 1240-1256.</ref> perhaps provide the closest modern analog to Merecure Unit A, although these modern systems principally carry boulders, gravel, and very coarse sandstone.

Table 1. Comparison of the stratigraphic nomenclature of this study with existing Corpoven sandstone nomenclature.

Table 1. Comparison of the stratigraphic nomenclature of this study with existing Corpoven sandstone nomenclature.

The sandstone is stored primarily in a well-defined north-northwest-trending axis that is approximately 6 km wide and splits into a second channel axis in the north part of the field study ([[:File:Methods14ch08f11.jpg|Figure 11]]). Sand-body geometry in the axial trend is low sinuosity, and the bedding architecture is aggradational, as indicated by the uniformly blocky gamma-ray and SP log patterns ([[:File:Methods14ch08f12.jpg|Figure 12]]). The sandstone-dominant, blocky bedding architecture is attributed to deposition from migrating transverse and longitudinal bar forms that are typical of sandy, braided river systems. The well-defined mud-rich areas of this interval are characterized by uniformly serrate log packages and are interpreted as interchannel floodplain facies.

The sandstone is stored primarily in a well-defined north-northwest-trending axis that is approximately 6 km wide and splits into a second channel axis in the north part of the field study ([[:File:Methods14ch08f11.jpg|Figure 11]]). Sand-body geometry in the axial trend is low sinuosity, and the bedding architecture is aggradational, as indicated by the uniformly blocky gamma-ray and SP log patterns ([[:File:Methods14ch08f12.jpg|Figure 12]]). The sandstone-dominant, blocky bedding architecture is attributed to deposition from migrating transverse and longitudinal bar forms that are typical of sandy, braided river systems. The well-defined mud-rich areas of this interval are characterized by uniformly serrate log packages and are interpreted as interchannel floodplain facies.

The external sand-body distribution and internal bedding architecture of the Oficina Unit B resemble that of a sandy, braided fluvial system that is confined by stable (vegetated?) floodplains. The modern sandy, braided William River in Canada ([[:File:Methods14ch08f13.jpg|Figure 13]]; Miall, 1992) is regarded as a possible analog.

The external sand-body distribution and internal bedding architecture of the Oficina Unit B resemble that of a sandy, braided fluvial system that is confined by stable (vegetated?) floodplains. The modern sandy, braided William River in Canada ([[:File:Methods14ch08f13.jpg|Figure 13]])<ref>Miall, A. D., 1992, Alluvial deposits, in R. G. Walker and N. P. James, eds., Facies models: Response to sea level change: Geological Association of Canada, p. 119-142.</ref> is regarded as a possible analog.

===Structural Complexity===

===Structural Complexity===

==References==

==References==

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==External links==

==External links==