Early Recognition of Seabed and Sub-Seabed Natural Hydrocarbon Seeps in Deep Offshore Angola
Authors: Unterseh Stephan Luis 
Published: June 2014

Natural hydrocarbon (HC) seeps are commonly observed at seabed and at shallow depth below seabed in the deep offshore Angola. They are associated to various types of geological features, such as pockmarks, Methane Derived Authigenic Carbonate (MDAC) crusts, nodules, mounds or more competent layers, asphalt nodules / mounds, and clay-oil mixtures. Those geological objects are considered as significant geohazards for any deepwater development, since they may affect and/or modify the soil geotechnical properties within the foundation depth (considered here as the top 100m below seabed). The early recognition of the areas affected by natural HC seeps is therefore a key milestone in a successful field development strategy.

The standard practice to detect natural HC seeps consists in performing AUV surveys to acquire high resolution geophysical data (bathymetry, side-scan sonar imagery, sub-bottom profiling and photographs) and seabed samplings. However, such dedicated surveys are usually performed in pre-feed / pre-project phases and any preliminary development works are performed using the available 3D exploration seismic datasets; which have a low detection capacity of these seepage features.

This paper presents correlation results between seismic echo-facies and attributes from 3D exploration seismic data and high resolution data acquired during recent AUV surveys carried out in deep offshore Angola. These results are used to recognize and try to characterize areas with potential natural HC seep occurrences, where no AUV data is available yet, in order to adapt the future development layouts.

Introduction

There is a growing demand to develop faster and better the new discoveries in more and more geologically complex areas. The preliminary development works are therefore started at early stages with limited information regarding the site geology and large uncertainties regarding the development concepts. Among these preliminary studies, regional assessments of the soil conditions and potential geohazards are generally conducted using only 3D exploration seismic datasets. The primary objectives of such regional assessments are to (1) estimate the typical soil conditions, (2) identify and delineate any potential geohazards, (3) validate the preliminary development layouts and, of top most importance, (4) identify any potential show stopper or " no go" areas.

This study focuses on the deep offshore Angola, a geohazards-prone area particularly affected by the common occurrence of natural hydrocarbon (HC) seeps. Natural HC seeps are associated to various types of geological features, such as pockmarks, Methane Derived Authigenic Carbonates (MDAC), asphalts nodules / mounds or clay-oil mixtures. They may have significant impacts on the soil geotechnical properties (Hill et al., 2011) and therefore can be interpreted as possible " no go" zones. The standard methodology to assess the potential for HC infiltration in an area has been developed by Thomas et al. (2011) and typically relies on the results from high resolution AUV and 2D UHR surveys (identification / mapping of the concerned areas) and dedicated geotechnical surveys (geotechnical characterization / ground-truthing). Obviously, the 3D exploration seismic is also able to detect large scale features (e.g. pockmarks, carbonate mounds…) but has a poor detection capacity for the small scale features (asphalts extrusions, oil-clay mixtures …) compared to higher resolution datasets.