Sequence stratigraphic analysis of Eridanos deltaic shallow gas-bearing deposits in North Sea

Abstract

Growing interest has been expressed the last decades regarding the Late Cenozoic, gas-bearing sediments of Southern North Sea Basin. Comprising the shallow subsurface of the Dutch offshore sector, these sediments have
been deposited by Eridanos fluvio-deltaic system, draining the Fennoscandian and Baltic shield through the present Baltic Sea. Three successful producing fields - A12-FA (2007), F02a-B-Pliocene (2009) and B13-FA (2011)- and
five under development have triggered the conduction of several studies in the offshore area. However the deltaic environment has been characterized as highly complex owing to its influence by the onset of Northern Hemisphere
Glaciation and thus the processes that governed the system have not become entirely understood. Enhanced cooling followed by the waxing and waning of the glaciers during warmer periods had an immediate impact on sediment
supply, accommodation space and mineralogical input. Although the sediments have been studied in terms of chrono-biostratigraphy, no systematic investigation with respect to the three main aforementioned factors as well
as the regional sequence stratigraphy and its link to the reservoir deposits has been made up to date. Therefore, this study employed sequence stratigraphy as a method to examine Eridanos conditions of deposition, investigate the
interplay between accommodation space and sediment supply and explore the nature of the gas-bearing reservoir sediments. The adopted methodology is comprised of two basic pillars, observation and interpretation. The first
records as clearly as possible the observations arise from the sequence analysis while the second extracts the meaningful information and interprets it in terms of temporal and spatial concepts. Using 2D seismic and well log data the basic observations were the stratal terminations, stacking patterns, seismic facies and the shoreline trajectory.
Seismic and well log interpretation showed that the delta experienced multiple events of sea level fall which forced the shoreline to regress basinward and caused sediment erosion or non-deposition. These events are bounded
by nine time-significant surfaces of subaerial unconformities which constitute the depositional sequences. Normal regression comprised the dominant depositional trend, combined with aggrading-prograding patterns, leading to
characteristic alternations of highstand and lowstand system tracts. Three main depositional environments which correspond to open marine turbidites (submarine lobes), delta front and delta top were identified from well logs,
core descriptions and the seismic facies analysis. The study suggests that the shallow gas is located in the alternations of silty-sandstones and claystones of the delta plain which comprise the vertical stacking of parasequence
topsets within the highstand- lowstand system tracts. Reconstruction of the relative sea level changes and sedimentation rates was made based on a technique introduced by this study. It uses the average thickness of each seismic
unit in order to interpolate time between the three known absolute ages obtained by the literature. The graphs showed that the accommodation space was generated by a low rate of sea level rise while sedimentation rates were
increasing over time. However the scarcity of time constrains in combination with the uncertainty in the estimation of seismic volumes resulted in a low resolution outcome. A comparison between the findings of this study and
those of the existing literature was made. The overall depositional trends and conditions seem to be in accordance with the other surveys. Nonetheless the proposed interplay between accommodation and supply can be assigned as
local since the examined area is limited compare to areas studied in literature. Limitations are traced in the quality of the 2D seismic data hampering the observation regarding the relation between the reflectors and the surfaces.
Consequently, the study offers an insight into the conditions under which the gas-bearing deltaic sediments were deposited and tries to place them in the established sequence stratigraphic framework. Also it provides information
regarding the distribution of the deltaic environments and identification of the reservoir rocks within each setting. The resulting interpretation can be used for prediction of the reservoir formations since the genetically-related
packages where they were identified are distributed in a predictable manner within a sedimentary basin.