Use of Seismic Attribute in Detecting Hydrocarbon Traps in Deeper Olpad Sands in Cambay Basin

Sucheta Mukherjee, Anirbid Sircar, Kavita Bhardwaj, and Pushpal Banerjee
Jubilant Energy, Noida, India

Exploration in Cambay basin in the recent past dwelled within the shallow reservoirs. The recent discoveries by various operators in deeper sands within Olpad level open a new chapter in the history of exploration in Cambay basin.This paper highlights the power of seismic attribute in delineating Olpad reservoirs using good quality long offset 3D seismic data.

Seismic attributes can be broadly categorized into three classes namely-horizon based attributes, sample based attributes and stratal-based attribute. Horizon based attributes are calculated within a specified time window. Horizon based attributes include coherency type attribute, ratio type and averaged attributes. Sample based attributes are calculated at a particular time of interest. Stratal amplitudes are a special type of horizon amplitudes .In specific zones within the horizons where it is difficult to track a particular event in the seismic data several strata are created within the horizon of interest. The strata follow the upper or lower horizons or are an average between the two. Depending upon the coherency of the amplitude strength within the zones several attributes can be developed for each of the strata.

For some of the wells in the Cambay Basin that reached upto the Olpad levels tracking the hydrocarbon bearing sands were difficult, as the reflectors could not resolve the sands. Horizon based attributes could give an idea about the zones but in a much wider sense and did not truly represent the hydrocarbon bearing sands. Sample based attributes gave an estimate of the events but could not track the path of the sands. Moreover, the sands are finely laminated within thick claystone. Attributes derived stratal slices overcame both the shortcomings and represented both the path and the presence of the hydrocarbon charged sands in the Olpad. RMS amplitude, Average energy, Average Magnitude (horizon Based attributes) Instantaneous phase, frequency, Half Energy, Sweetness (Sample based attributes) were applied to the slices which gave an idea of the extent of the hydrocarbon charged sands.

Presence of hydrocarbon increases the amplitude strength and decreases frequency content. Hence, sweetness attribute that is the combination of both the attributes gives a better picture. This method helped us to propose not only successful exploration wells but also gave us an idea about the extent of the pool.

Presentation GEO India Expo XXI, Noida, New Delhi, India 2008©AAPG Search and Discovery