AVO analysis aids in differentiation between false and true amplitude responses: a case study of El Mansoura field, onshore Nile Delta, Egypt

Mohamed Hussein,Ahmed Abu El-Ata,Mohamed El-Behiry

doi:10.1007/s13202-019-00806-2

Abstract

The seismic amplitude versus offset (AVO) analysis has become a prominent in the direct hydrocarbon indicator in last decade, aimed to characterizing the fluid content or the lithology of a possible reservoir and reducing the exploration drilling risk. Our research discusses the impact of studying common depth point gathers on Near, Mid and Far-offsets, to verify the credibility of the amplitude response in the prospect evaluation, through analyzing a case study of two exploratory wells; one has already penetrated a gas-bearing sandstone reservoir and the second one is dry sand, but drilled in two different prospects, using the AVO analysis, to understand the reservoir configuration and its relation to the different amplitude response. The results show that the missing of the short-offset data is the reason of the false anomaly encountered in the dry sand, due to some urban surface obstacles during acquiring the seismic data in the field, especially the study area is located in El Mansoura city, which it is a highly cultivated terrain, with multiple channels and many large orchards on the edge of the river, and sugar cane and rice fields. Several lessons have been learned, which how to differentiate between the gas reservoirs and non-reservoirs, by understanding the relation between the Near and Far-offset traces, to reduce the amplitude anomalies to their right justification, where missing of Near-offset data led to a pseudo-amplitude anomaly. The results led to a high success of exploration ratio as the positives vastly outweigh the negatives.

Highlights

60 Tcf of gas reserves has been discovered until now in the Nile Delta province (Nini et al 2010), from different stratigraphic levels, ranging from the Oligocene to the Plio-Pleistocene; it is considered the most prolific province for gas production in Egypt
At El Mansoura study area, the Neogene succession consists of the Messinian sandstones and shales of Qawasim Formation, which are unconformably overlain by the Pliocene shales with minor sandstones of Kafr El Sheikh and El Wastani Formations
The modeling and analysis of the CDP gathers on Near, Mid and Far-angles of the two drilled wells have been concluded that, there is a great variation between the results of the two prospects, even though the seismic characters of the amplitude anomalies in the stacked sections are similar to a great extent and have the same structure and stratigraphic sequence

Summary

Introduction

60 Tcf of gas reserves has been discovered until now in the Nile Delta province (Nini et al 2010), from different stratigraphic levels, ranging from the Oligocene to the Plio-Pleistocene; it is considered the most prolific province for gas production in Egypt. The hydrocarbon potential of the Nile Delta is believed to be limited to the Neogene–Quaternary sequence (EGPC 1994; Abdel Halim 2001). Many wells have been drilled targeting the Messinian section, where clear AVO class III anomalies indicate the presence of gasbearing sands within the Qawasim Formation. After drilled both anomalies, it has been found that the first well (Fig. 4, left) was gas-bearing sand and the second one (Fig. 4, right) was dry sand. This question cannot be answered before more investigation and studying the AVO response at the two drilled locations, to avoid any further dry wells in the future exploration and development of any other fields Why is there a difference in the results of the two wells? This question cannot be answered before more investigation and studying the AVO response at the two drilled locations, to avoid any further dry wells in the future exploration and development of any other fields

Methods

Results

Conclusion