The depositional model for the Tor Formation at the Valhall and Hod fields has been revised to reflect the influence of syn-depositional faulting and reworking on reservoir thickness variations. New ideas for the structural and depositional model were based on detailed reservoir interval correlations in the Valhall-Hod area using biostratigraphy and graphic correlation techniques. Three-dimensional seismic data was also integrated and applied to develop the model. The Tor Formation was deposited during a tectonically active time in the Late Cretaceous and the Early Tertiary. During this time Valhall and Hod anticlines were forming as a result of inversion along the Lindesnes Ridge. Regionally the Tor and Hod formations thicken away from the Ridge. This indicates that the area was a structural high which is interpreted to be a large shoal. A major unconformity is present at the Hod/Tor Formation boundary. During continued uplift, crestal areas collapsed forming a series of horsts and grabens. The Tor Formation (Campanian and Maastrichtian, with documented Danian reworking) filled in the lows formed by these graben. Seismic data and well control demonstrate that the Tor Formation is thicker in graben and thinner over horsts. Removal of the Hod and Tor formations from local structural highs can be best explained by wave action and winnowing across a large shoal. The graben areas, sheltered from waves and protected from scouring and erosion, became local depositional centres for thicker slumped and reworked Tor Formation chalk. In the Tor Formation there is strong evidence for local erosion, formation of hardgrounds, grain size sorting, and down-slope movement through slumping and debris flows. Thickness and facies variations in the Tor Formation of Valhall and Hod fields can be linked to syn-depositional faults. Recognition of a fault controlled depositional model along with detailed mapping of thick reworked chalk in local graben led to the definition of additional reserves in the Valhall-Hod area.

Palaeoenvironmental analysis in the North Sea chalk is hampered by the prevalence of widespread allochthonous units, intervals of massive reworking of microfossils and an often monotonous lithology of massive chalks of uncertain depositional history. Most difficult of all is devising precise regional biostratigraphic correlations in a complex section with large, but very subtly expressed, disconformities and condensed zones. Facies change quickly both laterally and vertically. Without very precise regional correlations, it is impossible to formulate any detailed palaeoenvironmental model. Through a multidisciplinary palaeontological approach, employing graphic correlation methodology and a composite standard database, a highly accurate and detailed regional chronostratigraphy was generated. Biofacies recognized within the section were then scaled in absolute time, allowing for more reliable recognition of facies trends at any one time horizon. Biofacies were formulated based upon recovered benthic foraminifera, planktic foraminifera, calcispheres, siliceous microfossils and macrofossil debris. Biofacies mapping for several higher-order sequences recognized in the Upper Cretaceous chalks has allowed recognition of four major periods of deposition: (1) early Turonian-early Santonian high pelagic productivity and rapid deposition of chalks over a relatively continuous ridge from the Valhall to Hod fields; (2) early Campanian-earliest Maastrichtian deep-water autochthonous chalk deposition off-structure with structural crests mantled by submarine hardgrounds; (3) very heterogeneous autochthonous and allochthonous middle Maastrichtian chalk deposition controlled by local grabens formed from crestal collapse; and (4) widespread sediment flows and redeposition of sediments with crestal shoaling and winnowing in the latest Maastrichtian culminating in crestal erosion of Cretaceous chalks in the early Danian.

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The Kapok Field is located some 37 miles southeast of the island of Trinidad, and consists of three separate offshore gas discoveries: namely Sparrow, Renegade and Parang (Fig. 1). Following discovery, four appraisal wells were drilled into the structures in the late 90s to test areas where seismic images were masked by shallow gas. The wells confirmed the presence of gas condensate and minor oil in a total of 18 sands. The development of the Kapok field presents some unique challenges for Trinidad with comparatively shallow multiple stacked pay, aerially extensive reservoirs, relatively thin columns underlain by large aquifers, and poor seismic imaging caused by shallow gas. To address these challenges a multidisciplinary reservoir description and stochastic modelling study was conducted to improve understanding of water behaviour - the key reservoir risk - and to plan optimum locations for the field development wells. The integrated workflow for the project incorporated seismic mapping, 3D structural modelling, the use of detailed core descriptions and field analogues, as well as both full field and single sand reservoir simulation. The development of Kapok will be implemented in two phases: Phase I will develop the reserves associated with Sparrow and Renegade, Phase II will target Parang. Phase I calls for 16 high angle/horizontal, openhole gravel packed wells and the installation of a new 20 slot normally unmanned, remotely operated platform. The wells, some capable of delivering 200+ Mmcfd, will initially deliver 1 bcfd for processing on the new Cassia ‘B’ Central Processing Unit (CPU), prior to delivery into the 48in Bombax pipeline and transportation to the Trinidad mainland. The Cassia ‘B’ CPU will have the capability of processing 1.6 bcfd of gas, 50 Mstbd of crude/condensate and 25 Mstbd of water from the existing Cassia, Immortelle and Flamboyant fields, as well as from Kapok and any future developments in the area. The CPU will be one of the largest offshore processing units in the world. With the Kapok platform and the Cassia Hub, BP is working to promote overall safety and environmental standards within Trinidad, with re-injection of produced water and drill cuttings disposal forming part of the enhanced environmental measures. Introduction The new subsurface challenges presented at Kapok called for an extensive subsurface evaluation, marking a step change from BP's prior Trinidadian developments. A significant amount of work went into front end loading the project with detailed efforts centred on in-depth structural modelling, stochastic geo-cellular modelling, full field and single sand simulation and focused Reservoir Value Assurance. There has also been the acceptance that water drive will at some point play either a beneficial or detrimental part in the ultimate behaviour of the sands - this has been incorporated into future performance predictions.

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