The Value of Onshore Geology for Caribbean Offshore Exploration: Walton Basin, Jamaica

Abstract

Tullow Oil is the operator and majority stakeholder in a 32,000 km2 frontier exploration licence encompassing the Walton and Morant Basins, offshore southern Jamaica. Since signing the block in 2014 exploration effort has been focused on developing play concepts and integrating them with interpretation of a mixed vintage (late 1970s — 2017) 2D seismic grid, with a current emphasis on the prospectivity of the central Walton Basin where 3D seismic is now being acquired. This Paleogene rift-sag basin inherited a Cretaceous structural template which was later modified and then reactivated by Late Neogene strike slip tectonics. Understanding this prolonged and varied tectonic history and resulting structural complexity is further challenged by imaging issues associated with acoustically hard sea bed and substantial sea floor topography, and a paucity of direct well calibration. Only one offshore well is located within the current Walton-Morant licence. Arawak-1 was drilled in 1982 and is situated on the Pedro Bank, a long-lived carbonate platform to the south of the Walton Basin. Log and cuttings data show that this is not fully representative of the basin succession. In contrast, late Neogene tectonic uplift of Jamaica provides direct access to pre-Miocene geology from onshore exposures and multiple boreholes. Outcrops are dispersed and correlation between them is not straightforward, however collaboration with local experts and critical evaluation of published data has maximised the value of the onshore work by allowing field and sampling campaigns to be focused on specific objectives to derisk the offshore plays. Onshore hydrocarbon shows in boreholes and at surface provide direct evidence of a working petroleum system. This is predicted to extend offshore into the Walton Basin where traps are less likely to have been breached during Neogene uplift. Understanding the Cretaceous — Palaeogene succession in terms of provenance, sediment routing, depositional palaeoenvironments and structural trends permits extrapolation and prediction of the presence and distribution of source rocks, reservoirs and seals to the subsurface offshore. Tullow’s exploration strategy has been to combine evaluation of key petroleum system elements from onshore fieldwork and well sampling with the offshore geoseismic interpretation, synthesised within a framework of the regional plate tectonic history to maximise exploration effectiveness. The Walton Basin is a Paleogene rift — sag basin that developed on a precursor Late Cretaceous structural template. The presence of Albian — Campanian marine intra-arc rifts is evidenced from onshore exposures of slope-basin hemipelagic mudrocks, volcaniclastic turbidites and debrites, with discrete episodes of carbonate shelf development that provide a framework for correlation between different tectonic inliers. Some of the Cretaceous mudrocks have elevated TOCs and hydrocarbon generating potential, albeit reduced by weathering at outcrop, consistent with regional observations. Latest Cretaceous facies indicate shallowing and a transition to marginal marine environments in which additional organically-rich but lignitic source rocks were locally deposited. The shallowing accompanied a mild inversion episode when the Caribbean Arc that underlies the Jamaican part of the Northern Nicaragua Rise (NNR) collided obliquely with the Yucatan terrane of the North American plate. Although seismic attenuation in the thick and carbonate-rich Tertiary overburden hampers Cretaceous basin mapping, published geophysical data support the presence of discrete troughs below the Walton Basin, and these are likely to have a comparable fill to the onshore correlatives. The Palaeocene to Middle Eocene succession on Jamaica records syn-rift to early post-rift deposition, with progressive marine transgression punctuated by episodic but diminishing tectonic activity. An Early Palaeocene coarse alluvial fan / fan delta and volcanic / volcaniclastic succession is exposed in the NW-SE Wagwater Trough rift. Offshore chaotic high amplitude seismic facies that form acoustic basement in much of the fault-bounded Walton Basin depocentres are inferred to be coeval and analogous. In the Early Eocene the Wagwater Trough accumulated shallow then deep marine sediments, dominated by turbidites with palaeocurrent trends indicating substantial transfer of sand into the offshore basin area. Despite mineralogical immaturity the Early Eocene sandstones are clean and well sorted and may host secondary porosity in offshore slope apron and fan deposits. Coeval deep marine shales in the deepest rift settings are predicted to contain source rock facies. Coeval horst blocks on Jamaica record shallow marine to terrestrial volcaniclastic facies. Localised but subdued tectonic activity continued into the Middle Eocene and shallow marine to coastal sandstones, shales and limestones were deposited across Jamaica. Post- rift topography, eustasy and autogenic processes resulted in a cyclothemic and laterally variable succession of deltaic to estuarine facies. Sandstone petrology from fieldwork in combination with published palaeontological evidence indicate that Jamaica was connected to the North American plate at this time, and southerly-directed palaeocurrent indicators allow prediction of shelfal to submarine fan reservoirs in the offshore basins. Texturally mature, quartz-bearing litharenite sandstones were derived partly from felsic crustal protoliths north of Jamaica and partly from local sources and are predicted to form high quality reservoirs offshore. Coeval lagoon / embayment shales are rich in Type II/III kerogen, and include substantial liptinic intervals with oil and gas generative potential. By analogy with Eocene paralic systems in other petroliferous basins the correlative delta front to basin shales in the Walton Basin are likely to be organically rich and oil-prone. Continued transgression in the later Middle Eocene coincided with undocking of the NNR from the Yucatan terrane as the Caribbean plate moved eastwards and the Cayman Trough was initiated. Onshore Jamaica, the diminution of siliciclastic input resulted in deposition of marine shales interspersed with tight muddy limestones and marls. These are likely to continue into the Walton Basin and provide a top seal for the Middle Eocene sandstone reservoirs. Based on Arawak-1, carbonate deposition on the Pedro Bank may have begun earlier, influenced by pre-existing topography. This implies that the Walton Basin acted as a siliciclastic sediment trap. From the Late Eocene until Early Miocene carbonate deposition was established across Jamaica. Shallow marine open platform facies accumulated on former horsts and deeper marine periplatform turbidites and chalks were deposited in residual fault-bounded troughs. Stratigraphic intercalation of porous outer platform shoal facies with tight inner platform lagoonal facies probably reflect a 3rd order eustatic signal, and may provide stacked reservoir-seal pairs in parts of the Walton Basin. Seismic evidence raises the possibility of reservoir augmentation by fault-related diagenesis, and onshore boreholes provide evidence of seawater dolomitisation creating zones of excellent reservoir quality. The centre of the Walton Basin, away from basement highs, was probably a periplatform to basinal carbonate environment for much of the time. Relative tectonic quiescence during the Late Eocene to Early Miocene accompanied the steady eastward migration of the Caribbean plate. Regional subsidence associated with crustal thinning in the adjacent Cayman Trough, driven by a regional transition to a dominant strike slip regime, provided accommodation space for carbonate aggradation. However, in the Middle Miocene a change in the plate boundary kinematics and formation of the Gon‚ve microplate led to concentration of strike slip displacement on the Enriquillo- Plantain Garden fault through central Jamaica, and formation of a restraining bend in the Blue Mountains area. Resulting uplift, emergence and erosion of present day onshore Jamaica was accompanied by further subsidence of the Walton Basin, transtensional or strike-slip reactivation of some of the Palaeogene faults, and localised carbonate platform drowning. However, the more distal Pedro Bank continued to aggrade and develop its modern steep-sided topography. Late Neogene and Quaternary sedimentation in Jamaica is confined to the modern coastal zone and comprises nearshore shallow marine carbonate facies interspersed with terrigenous siliciclastics derived from rivers draining the uplifted Blue Mountains, the inverted Wagwater Trough and other highland areas. With no major siliciclastic delivery systems feeding into the Walton Basin, and only modest strike-slip displacement on its northern margin, deposition was dominated by in-situ and reworked deep marine hemipelagics, interspersed with calciturbidites and probable mass wasting deposits sourced from the margins of the Pedro Bank. Understanding the onshore geological history has allowed Tullow to propose two offshore petroleum systems, with Palaeogene siliciclastic and carbonate reservoirs, charged by Palaeogene and/or Cretaceous source rocks. The Palaeogene-Palaeogene petroleum system is well defined but ineffective onshore owing to Miocene uplift preventing maturation of the organically rich Middle Eocene source rock. It is predicted to be present and effective in the Walton Basin that continued to subside through the Neogene. In contrast, the Cretaceous- Palaeogene petroleum system is active onshore, contributing to the oil shows in boreholes and at outcrop but compromised due to breach of traps during Miocene inversion. It is likely also present in the Walton Basin where trap integrity is less of a risk, although Cretaceous kitchens are difficult to image on the seismic data. Either of the petroleum systems may have contributed to a live oil seep recently documented by Tullow on the margin of the Pedro Bank that is typed to a mature Cretaceous or Tertiary marine source rock. The location of this seep is compatible with up-dip migration from the Walton Basin.

AAPG Datapages/Search and Discovery Article #90330©2018 AAPG Hedberg: Geology of Middle America – the Gulf of Mexico, Yucatan, Caribbean, Grenada and Tobago Basins and Their Margins “Integration of the geology of the area from the southern onshore of North America to the northern onshore of South America”, Siguenza, Guadalajara, Spain, July 2-5, 2018