PGK - Petroleum Geologische Kring

Accurate rock property prediction is often a critical success factor for wells targeting hydrocarbons. This applies not only to reservoir porosity and permeability affecting productivity directly, but also to acoustic velocity, seismic time-to-depth conversion and depth prognosis. A detailed analysis of variation in the overburden rock velocity in the Southern North Sea has shown that Triassic velocity variations of up to 18% occur within short distances (e.g. <1 km). A correlation was found between increased acoustic velocities and the presence of an underlying salt weld. Salt Induced Stress Anomaly (SISA), a geomechanical model, is presented that can explain these observations and is based on the principles of pointloading and buoyancy. In the initial state, prior to salt movement, the vertical effective rock stresses resulting from the overburden weight are transmitted uniformly and cause laterally even compaction in the sediments. However, once the salt layer is able to flow and redistribute itself under the influence of buoyancy forces, the overburden stress will concentrate itself near the salt welds. This locally increased stress gives rise to higher velocities in the overburden. The same stress concentration model can also explain deterioration in the porosities of the Rotliegend reservoir as observed underneath salt welds.

The Southern Permian Basin (SPB) formed a large, elongated saucer-shaped inland depression extending from theUK to Poland and from Belgium to Denmark. During the Middle to early Late Permian the SPB was filled progressively by playa sediments from its centre in northwestern Germany before it was flooded at the onset of the Zechstein. The Netherlandswere were situated at the southern part of the saline playa lake.

The sediment fill has an onlap configuration upon the underlying Carboniferous and Lower Rotliegend strata. Althoughregional contour maps suggest a fairly gradual thinning of the Upper Rotliegend interval towards the basin edge, more carefulexamination reveals that there are prominent steps in thickness and facies. It can be concluded that these steps are defined bysteps in the palaeotopography of the basin.

These topographic steps are related to pre-Variscan and Variscan structural elements. During deposition of the UpperRotliegend these large-scale topographic steps defined the location of the main fluvial fairways towards the basin centre,extensive (mud)flat areas, and the relative highs on which the dune fields developed. Next to these large topographic elementsrelief inversion, differential erosion, and faults caused smaller-scale topographic relief elements. This relief exerted prominentinfluence on the sediment dispersal patterns and sediment type both laterally and vertically. This holds especially for the basalRotliegend sandstones and in the pinch-out area near the playa margin.

Electromagnetic (EM) methods have been used in exploration for a long time, even before seismic. In the early days of exploration Magneto-Telluric (MT) methods have been employed in remote areas as in the field only receivers were required. The source used was either the EM fields generated by the sun or in the ionosphere. In well logging EM is used in induction tools. Recently Controlled Source EM (CSEM) is seen as a way of obtaining independent information on the reservoir fluid. With the receivers the electrical resistivity of thin layers in the subsurface can be obtained. Hence, in addition to seismic information to delineate the geological structure the EM based methods provide the pore-fluid information.

Whereas seismic is based on wave propagation, EM is based on diffusion, or can it also be seen as wave propagation? Just as light can be described by wave propagation along rays as well as by wave-fronts, whatever is most appropriate and convenient, so can EM in the Earth also be described by diffusion as well as propagating "waves", be-it severely attenuated.

In MT often Transverse Electric (TE) and Transverse Magnetic (TM) data are mentioned. The origin of that terminology is related to MT acquisition along seismic lines, which can be acquired in dip direction (to be preferred) or strike direction. MT is often associated with investigating conductivity and CSEM is mainly used for mapping resistivity. It will be shown that combining inductive MT and galvanic CSEM data provide the best resistivity subsurface model and certainly in combination with seismic data for structural information.

However, these various kinds of independent data sets need to be combined. This can be done by sequential inversion but the best way is by joint inversion whereby each data set can be weighted by its estimated accuracy. Results of recent inversions, also including gravity information will be shown.

Late Eocene sea retreat from the Tarim Basin (west China) and concomitant Asian paleoenvironmental change
Roderic Bosboom (Utrecht University)

The Paleogene sediments of the Tarim Basin in western China include the remnants of the easternmost extent of a large epicontinental sea, which extended across the Eurasian continent before it retreated westward and eventually separated as the Paratethys Sea. Climate modeling results suggest that this sea retreat is an important forcing mechanism in the aridification of the Asian continental interior. However, the age and paleogeography of the retreat are poorly constrained, hindering understanding of its cause and impacts. Our research reports litho- and biostratigraphic results from various sections recording the last major regression out of the Tarim Basin. Rich micro- and macrofossil assemblages (including benthic foraminifera, ostracods, bivalves, calcareous nannofossils and dinoflagellate cysts) indicate a shallow, proximal and marine environment and date the last marine sediments as latest Bartonian to earliest Priabonian in age (~37 Ma), time-equivalent to the sea level lowstand at the Bartonian–Priabonian boundary but pre-dating both the Oligocene–Miocene regional uplift of the Pamir and Kunlun Shan and the
major eustatic sea-level falls of the Eocene–Oligocene Transition (~34 Ma) and mid-Oligocene (~30 Ma), which are usually held responsible for the sea retreat. Furthermore, a concomitant and significant aridification step at ~36.6 Ma recorded in the Xining Basin along the northeastern Tibetan Plateau suggests that the Tarim Sea served as a significant moisture contributor for the Asian interior.

Relating Petroleum System- & Play Development to Basin Evolution
Suzanne E. Beglinger, Maarten P. Corver, Harry Doust & Sierd Cloetingh (VU Amsterdam)

Most oil and gas provinces are becoming increasingly mature with respect to exploration. Therefore, our understanding of petroleum prospectivity of less well explored basins becomes more important. In such areas, the use of mature basins as analogues can contribute to the identification of new hydrocarbon discoveries. We anticipate that the use of analogues will become increasingly valuable in exploration. The objective of this research is to increase the knowledge of interpreting basin data in order to facilitate prospectivity prediction in new venture exploration through the recognition of patterns of petroleum system- and play development in basin (cycle)s with similar  tectonostratigraphic characteristics.
We demonstrate a newly developed method, allowing for more efficient comparisons between basins with a similar geological background, i.e. basins which could act as analogues for exploration purposes. We apply the concept that sedimentary basins can be classified according to their structural genesis and evolutionary history. The classification is based on breaking the basins down into their tectonostratigraphic cycles. Common elements of petroleum systems and plays are linked to these natural cycles in order to improve the prediction of potential petroleum systems and plays occurring in under-explored basins.

Full elastic seismic data modeling of an outcrop-based high-resolution geological and petrophysical model, Book Cliffs (Utah, USA)
Daria Tetyukhina, Stefan M. Luthi and Dries Gisolf ( Delft University of Technology)

Outcrop observations are often used to develop predictive models and provide quantitative parameters describing the architecture and facies distribution of shallow-marine deposits at a sub-seismic scale to aid the development of exploration and production strategy. The focus of this project is to create a high-resolution depositional model that provides realistic geological and petrophysical reservoir information. As a basis for our geological and petrophysical modeling the analog data from the Book Cliffs (Utah) was used. The model is populated with shear velocity properties using regressions based on empirical data and laboratory experiment. The resulting realistic high-resolution geological model was used to create a seismic synthetic full elastic data-set that bridges the gap between seismic and analog data. The predicted seismic response allows to asses the appearance of small scale stratigraphic features on the seismic data. The ’Kennett invariant embedding method’ was employed for a forward modeling. The method is exact and calculates all internal multiple reflections and transmission effects, as well as all mode-conversions from P to S and vice-versa. Another potential application of the resulting seismic synthetic full elastic data-set is that the full wave form seismic (linear and non-linear) elastic inversion techniques can be applied/ tested. This inversion can provide insights into understanding and interpreting which geological features can/cannot be resolved from seismic data.