Lower Miocene Sequence Architecture in the Central Swamp Depobelt of the Niger Delta: Impact on Production

Abstract

Abstract The Lower Miocene strata of the gas-rich X, Y and Z oil fields in the Central Swamp depobelt of the Niger Delta, is evaluated in terms of its sequence stratigraphic architecture. The penetrated interval includes three superimposed estuarine valley-fill sequences (S1: 17.7Ma-16.7Ma; S2: 16.7Ma-15.5Ma; S3: 15.5Ma-13.1Ma). Apart from Field Z where S3 includes a lowstand systems tract, each valley-fill contains a fluvio-estuarine transgressive systems tract (TST) and a highstand systems tract (HST). This stratigraphic architecture appears to have a strong influence on reservoir properties and petroleum resource distribution. The bulk of the total original oil in place in Field X and Field Y, which occurs in narrow, wedge-shaped, discrete/amalgamated, proximal fluviatile-distal marine sand bodies encased in marine shales, are restricted to the 17.7Ma-17.4Ma transgressive estuarine valley fill of S1. In Field Z, the productive reservoir consists of alternation of detached shoreface sand lobes and outer neritic-bathyal shale. This reservoir occurs within the 15.5Ma-15.0Ma lowstand and transgressive systems tract of S3. Highest quality and greatest degree of homogeneity are indicated for Field X and Field Y reservoirs. Sand body geometry and heterogeneity in internal architecture of Field Z reservoir account for the more complex lateral and vertical fluid flow reported from this field. The new knowledge of the depositional characteristics of the reservoirs and the genetic relationship among the oilfields minimises geological and dynamic uncertainties in the inter-field development plan. Introduction The sequence stratigrphic concept provides an excellent tool that addresses the problems of genetic relationships among oilfields and reservoirs, as well as sand development and property distribution. This report which focuses on the Central Swamp depobelt of the Niger Delta, evaluates the sequence stratigraphic architecture of X, Y and Z oilfields (Fig. 1). The three fields were discovered in 1971, 1966 and 1961 respectively. Field Y is separated from Field X to the north, by a major fault and from Field Z to the south by a macrostructure-bounding fault. Each of the three fields contains one major oil reservoir (Field X-E1, Field Y-D7 and Field Z-E2 reservoirs respectively) at depths between 10,000ft and 12,000ft below the surface. The genetic relatioships of the three fields has remained a matter for speculation. Water injection was started in Field Z in 1981, but was discontinued in 2001 due to inexplicable water losses and persistent steep drop in pressure. To account for this, some schools of thought have speculated that the three major reservoirs might possibly be connected. It was therefore imagined that if production in the more proximal Field X and Field Y caused pressure drop in the more distal Field Z, a common production strategy could be adopted for the three fields.