First Steps Toward Maturing the Shallow Gas Play - Results of an Integrated Exploration Workflow
Authors: Ten Veen J. H. , Verweij J. M. , de Bruin G. , Donders T. 
Published: March 2015

Recent exploration activities in two of the largest deltas in the world, the still active Nile delta and the Cenozoic Southern North Sea (SNS) deltas, proved the potential of shallow gas resources. Although, previously seen as a hazard or an exploration tool for deeper hydrocarbons, the shallow gas accumulations may represent a valuable additional hydrocarbon resource, especially if located near existing infrastructures. Nonetheless, shallow gas production is still limited due to a lack of insight in the petroleum system. Knowledge on the geological conditions that enable accumulation of shallow gas is essential since gas at these depths is highly buoyant, and tends to migrate towards the surface. Furthermore, the nature of these accumulations depends on the type of sediment and the anatomy of the delta they reside in. In order to mature the shallow gas play, a multidisciplinary workflow was applied to the SNS delta that involves 1) the reconstruction of the internally complex delta body, 2) a combined deterministic/stochastic approach to make reservoir property predictions, 3) evaluation of the HC origin, and 4) a grain-size based method to predict the seal-integrity of the sealing clay layers. The results include a first evaluation of the potential of shallow HC accumulations in terms of trapping geometry, seal capacity, sourcing and migration. The presented workflow is applicable to areas where limited exploration data is available, but where critical production data is (still) missing. By reviewing the HC systems of the Nile and SNS deltas many similarities emerge that are expressed by 1) the control of sea-level and climate on the distribution of reservoirs, seals and organic material, 2) the presence of stratigraphic traps and 3) the role of deeper salt and faults in the formation of structural traps. For both settings, the origin of the shallow gas may be deep subsurface thermogenic sources or biogenic sources in shallower strata, or a mixture. To date, reserve estimates for the shallow gas play are often hard to make using conventional exploration techniques due to the inability to discriminate high vs. low saturation shallow gas. The potential of pre-stack seismic inversions or other geophysical techniques such as CSEM appear essential in maturing the Shallow Gas play.

Introduction

In the Southern North Sea, shallow gas is defined as gas that resides in shallow marine to continental (deltaic) deposits of the Plio-Pleistocene Southern North Sea (SNS) delta (Fig. 1). It is either structurally trapped in anticlines above salt domes, associated with lateral fault seals, or occurs in stratigraphic or depositional traps (Fig. 2). Traditionally, shallow gas occurrences were regarded as hazardous or non-economic because of low gas saturations ("fizz gas"). The location and size of the shallow gas fields are generally determined from seismic surveys, where sediments with a gas content of a few percent and more appear as areas of high reflectivity ("bright spots"). Kuhlmann and Wong (2008), linked the occurrence of potential gas (or rather bright spots in seismic data), to specific delta sub environments and stratigraphic intervals related to the inception of Late Cenozoic glaciations. This study was instrumental in illustrating the importance for exploration and production to understand the regional extent and characteristics of the delta. Stuart and Huuse (2012) focus on the interpretation of several bright spots in terms of fluid and lithological content through well log and seismic facies analysis in order to discuss the origin of these features.

None of these previous studies, however, successfully determined the relation between bright spot properties and the volume of shallow gas contained in a trap. This especially applies to the estimation of gas saturations. Therefore, based on the current state of knowledge, it is difficult to quantitatively estimate the volumes contained in shallow gas prospects. At present, several gas fields are produced, despite the fact that shallow gas forms a production challenge for exploration and production because of expected sand production and early water breakthrough in highly permeable and unconsolidated sediments.