--> EXTENDED ABSTRACT: 4-D Understanding of the Evolution of the Penal/Barrackpore Anticline, Southern Sub-Basin, Trinidad, by Moonan, Xavier R.; #90135 (2011)

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4-D Understanding of the Evolution of the Penal/Barrackpore Anticline, Southern Sub-Basin, Trinidad

Moonan, Xavier R.1
(1)Exploration & Geophysics, Petrotrin, Pointe-a-Pierre, Trinidad and Tobago.

The Penal/Barrackpore Anticline is a sub-surface south easterly verging detached Middle Miocene frontal fold within the Southern Basin Trinidad, generated by the south eastward directed oblique collision of the Caribbean Plate/Accretionary Prism with the northward subducting South American Continental Crust. (Fig.1) The oblique collision generated foredeep settings in the Southern Basin, into which the Middle Miocene primary reservoir, upper bathyal syn-kinematic Herrera sand turbidites were deposited in synclinal lows. (Fig.2). The Penal/Barrackpore Oilfield spans at least 17km long by 5km wide, with the anticline achieving a structural relief of greater than 6000ft (Hosein 1990, Dyer 1992, Telemaque 1996).

Penal/Barrackpore Anticline 3D Static Model

The integration of well data, and semi-regional 2D seismic lines across the Southern Basin coupled to surface geology revealed at least three structural levels within the Penal/Barrackpore Anticline, namely the Overthrust, Intermediate or Overturned limb, and Subthrust (Bitterli 1958, Dyer 1992, Ramlackhansingh 2007) (Figs.3&4). Pliocene gravity-driven extensional tectonics produced numerous north-west to south-east trending, eastward facing curvilinear detached normal faults which further dissected the structure by reutilising Middle Miocene syn-thrust extensional faults (Pindell 2005).

However, seismic data confirms that the westward facing curvilinear syn-thrust extensional fault (Main Extensional 2) was not reactivated during the Pliocene and as such serves as a migration pathway for hydrocarbons into the Middle Miocene reservoirs but more importantly separates the developed field from the relatively unexplored south-western plunging nose of the anticline (Figs.5&6). Pleistocene near-normal contractional/transpressional deformation resulted in refolding and retightening of structures producing Out-of-Sequence thrusts, backthrusts and mud diapirism (Pindell 2005, Ramlackhansingh et al 2009). The style of deformation during the Pleistocene may be directly related to the effectiveness of the underlying Lower Cretaceous/Jurassic? evaporite decollement as it thins out of the towards the Present Day South Coast of Trinidad.

4D Evolution and Restoration of Middle Miocene Turbidite Sand Fairways

4D evolution of the anticline, acquired by integrated restorations of 2D seismic lines and infill wells, revealed the growth of the anticline from a Lower Miocene detachment fold through to a Middle to Upper Miocene tri-shear fault propagation fold (Figs.7a-d). Restoration data, presented in the form of a Middle Miocene Vector Map demonstrates the disparities in the distribution of shortening over the Study Area from east to west (Fig.8). An observed NW-SE trending narrow elongated zone of reduced shortening labelled Z separates zones of more shortening (A & B), and coincides with the syn-thrust extensional fault, possible tear fault, named Main Extensional 2 and represents the in-plane impact of the fault on the distribution of shortening, amounting to a relative decrease of 1500m within the Penal/Barrackpore Anticlinal structure.

By combining the restored Middle Miocene topography, restored Net Sand values and palaeo-bathymetry, sand depositional modelling of the primary reservoir was undertaken, revealing two major north east to south west sand trends and their unique relationship to growing structures (Figs.9-11). Zones of absence of Middle Miocene Herrera sands coincided with palaeo-highs on the restored Middle Miocene topography. The northern sand trend averaged 150ft thick and was more confined (12km long x 2km wide) with higher net-gross ratios comparatively to the southern sand trend (9km long x 3km wide) averaging ~300-400ft thick, thinning southward to ~200-150ft. A narrow sand trend (2.0km long x 0.8km wide) orthogonal to the anticlinal axis appears to connect the northern and southern perched sand bodies.

Zones with a high probability of thick sand deposits were forward modelled to their Present Day locations, evaluated and ranked based on the geometry and nature of the structural or stratigraphic trap, as well as the possibility of tertiary hydrocarbon migration due to Pleistocene deformation in an attempt to de-risk future exploration drilling (Fig.12).

Conclusion

The Penal/Barrackpore Anticline is a subsurface south easterly verging fault propagation style fold with at least three structural levels namely the Overthrust, Intermediate or Overturned Limb, and the relatively undeformed Subthrust. Its Present Day configuration is the result of episodic contractional deformation due to the oblique collision of the Caribbean Plate/Accretionary Prism with the northward subducting South American Continental Crust culminating in the Middle Miocene producing an asymmetric foredeep into which the syn-kinematic Herrera sand turbidites were deposited in synclinal lows.

Restorations depicted the evolution of the Penal/Barrackpore anticline from a Lower Miocene buckling detachment folded foredeep, through to a Middle Miocene early stage fault propagation fold, and further through to a late stage fault propagation fold with shortcut thrusts into the Upper Miocene. A weak but positive relationship exists between the Restored Middle Miocene topography and the known distribution of Middle Miocene syn-kinematic turbidite Herrera sandstones. Simulated modelling of the syn-kinematic turbidites highlighted possible zones of thick sand deposits, some of which after forward modelling should occur within the highly prospective Intermediate Limb. Pliocene NW-SE trending cross cutting tear faults, some of which may have had a Middle Miocene syn-thrust extensional origin, appear to have a major role in oil accumulation along the anticlinal axis within the developed field and as such Main Extensional 2 fault may serve as a major trapping mechanism separating the relatively unexplored south-western plunging nose of the anticline from the developed field structurally updip.

3D seismic data is crucial for further understanding the lateral extent and behaviour of the tear faults/syn-thrust extensional faults identified as they will play a key role in the exploration drive to extend the Penal/Barrackpore Field to the south west.

Key References

Bitterli, P., 1958, Herrera subsurface structure of Penal Field, Trinidad, B.W.I. AAPG Bulletin, v. 42, n.1, p. 145-158.

Dyer, B.L. & Cosgrove, P. 1992. Penal/Barrackpore field, West Indies: South Trinidad Basin, Trinidad. American Association of Petroleum Geologists, Treatise of Petroleum Geological Atlas of oil and gas fields, p.139–157.

Hosein, F. 1990, Exploitation of the Middle Miocene oil bearing Intermediate Herrera Sandstone reservoirs in Trinidad, West Indies, in Transactions of the 12th Caribbean Geological Conference, p. 430 - 443.

Kugler, H., 1961, Geological Map and Sections of Trinidad, 1:100,000 – Orell Fussli. Zurich.

Pindell, J. & Lorcan, K., 2005, Paleogeographic Mapping and Documentation of Sandstones/Reservoir Systems, Palaeogene to Mid-Miocene of Trinidad and Eastern Venezuela.

Ramlackhansingh, A. 2007, Tectonostratigraphic Evolution of the Greater Trinidad Area (Onshore and Offshore): Exploration & Geophysics Group, Petrotrin Internal Report.

Ramlackhansingh, A., Ramsook, A., Boodoo, S., & Moonan, X., 2009, Exploration Evaluation of the Lower Cruse Formation within Petrotrin’s License Areas A&C, Southern Sub-Basin, Trinidad: Exploration & Geophysics Group, Petrotrin Internal Report 10355.

Saunders, J. B. 1974. Trinidad Stratigraphic Chart. Ministry of Energy of Trinidad and Tobago.

Telemaque, C., 1996, Petroleum potential of the Late Miocene and younger formations: Shallow Horizons Project, Petrotrin Internal Report.

Figure 1: Tectonic Map of Trinidad highlighting the Study Area (modified from Pindell, 2005).

Figure 2: Existing Model - Seismic Interpretation Line 177 (adapted from Ramlackhansingh, 2007) with Stratigraphic Column (adapted from Saunders, 1974).

Figure 3: Surface Geology of Study Area, Main Oilfields and 2D Seismic Lines (modified Kugler, 1961).

Figure 4: Annotated Seismic Interpretation along Dip-Oriented Line 177.

Figure 5: Seismic Interpretation of Strike-Oriented Line 321.

Figure 6: 3D Static Model of the Penal/Barrackpore Anticline highlighting zones of absence of the Middle Miocene Globorotalia fohsi lobata/fohsi (Gr7bc) Herrera reservoir.

Figures 7a-d: General Restoration steps for Line 185 using Midland Valley’s 2D Move “Move On Fault”.

Figure 8: Middle Miocene Globorotalia fohsi lobata/fohsi (Gr7bc) Vector Map of Study Area.

Figure 9: Middle Miocene G. fohsi lobata/fohsi (Gr7bc) Restored G. peripheronda (Gr 7d) Palaeo-Surface.

Figure 10: Well Data Constrained Middle Miocene G. fohsi lobata/fohsi (Gr7bc) Net Sand Map.

Figure11: Simulated Middle Miocene G. fohsi lobata/fohsi (Gr7bc) Net Sand Map.

Figure 12: Prospective Middle Miocene Herrera Sands, Restored Well Picks and Structural Zonation.

 

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.