Utilization of seismic attributes for reservoir mapping: A case study from the Cambay Basin, India

Multilayered Hazad sands of Middle Eocene age deposited in a deltaic environment are the main hydrocarbon producers in the south Cambay basin, India. These sands are broadly divided into 12 units (1 to 12) from bottom to top. These individual sand units are further subdivided into smaller subunits which are selectively charged and produce hydrocarbons in different parts of the south Cambay Basin. The study area covers a part of the Gandhar oilfield situated in the Broach-Jambusar block of the basin where subunits 3A and 12A have produced oil and gas in commercial quantities. Due to its widespread deposition and greater thickness, unit 3A was delineated and developed, based on conventional interpretation of 2D seismic data. The development strategy for unit 12A could not be completely resolved in the absence of a precise sand geometry map. A 3D seismic survey was carried out in the study area with one of the objectives being to map the lateral extent of thin pay sands exactly. According to the well data, the thickness of unit 12, consisting of subunits 12A, B and C, varies from 0 to 18 m, and wherever unit 12 is thicker, subunit 12A is present. Unit 12 is not resolved in the available seismic data but its seismic response is detectable. Synthetic seismic modelling has been carried out for a better understanding of the seismic response and for precise calibration. The seismic attributes extracted from the 3D data volume corresponding to unit 12 are utilized to generate a net thickness sand map of this unit using an artificial neural network technique. The sand geometry map of subunit 12A was prepared with the help of a net thickness map of unit 12 and well data. The thickness map of unit 12 helped in mapping the precise sand geometry of unit 12A and also an additional area for cost-effective exploration and development of this pay, which in turn improves the in-place reserves.