Third EAGE Exploration Workshop

Stochastic inversion of seismic data is being increasingly used as a tool for reservoir characterization. Indeed, it provides higher-resolution information on the reservoir at a much thinner and detailed scale than deterministic inversion and accounts for multiple scenarios of the same probability. However, it usually only leads to the elastic properties of the reservoir. These properties cannot be directly integrated into the petrophysical models used by oil and gas companies for reserve estimation and flow simulations. In order to address this limitation, this work describes a case study where a standard workflow is implemented to simulate and synthesize multiple porosity models using the results of seismic stochastic inversion, in addition to well data. The workflow relies on performing a suite of porosity simulations, using successive acoustic impedance realizations as soft data. For this purpose, a collocated cokriging sequential Gaussian simulation algorithm (more often called “co-SGS”) is used. The method produces much more satisfactory results than simply using linear relationships between porosity and acoustic impedance, because it ensures perfect well matching and accounts for data variability.

In 2010, Qatar Shell Upstream International B.V. (QSUI) re-entered Exploration in Qatar focusing on the relatively deep conventional Pre-Khuff (Paleozoic) gas plays with a view to discover additional hydrocarbons in the State of Qatar. The Block D license covers an onshore and shallow offshore/transition zone area of 8,089 km2 in the North-East of the country (Fig. 1). The Pre-khuff plays pose a key geophysical challenge of being able to image the Pre-Khuff section and hence assess trap materiality and configuration. Significant uncertainty exists in relation to the deep structure of the Qatar Arch particularly when in the region the Pre- Hercynian package has been shown to be structurally different to the overburden. To address these uncertainties, 2D seismic reprocessing and specifically targeted 2D/3D seismic acquisition have been deployed to improve elucidation of the structural configuration. These data demonstrate unambiguously the change in the dips of geological intervals below the Hercynian unconformity compared to the overlying Khuff and younger sediments. Regional 2D seismic lines, in conjunction with gravity anomaly data, are used to investigate the root structure of the Qatar Arch and the trap styles in the Pre-Khuff. The Qatar Arch is shown to be underlain by a low density Pre-Cambrian sediment filled graben. This graben was inverted through a number of mainly compressive tectonic events from Pre-Cambrian to present, resulting in the presently observed structural high of the Qatar Arch. Low amplitude thrust cored folds developed over the crest of the arch in response to these tectonic events and constitute the main prospective traps in the Pre-Khuff. The fault framework is assessed based on the combined analysis of gravity anomalies and seismic data and multiple sets of faults are identified both in the Pre-Khuff and Khuff/Post-Khuff sections. Integrated charge evaluation shows that the Qusaiba source rock over the Qatar Arch is presently in the mature gas window moving to over mature. Simple stress modeling on the mapped Pre-Khuff faults shows that a majority of them could be prone to re-activation in the present-day stress field and could therefore be leaking charge into the prospective reservoirs.

Recent discoveries in North and South Kuwait have upgraded the hydrocarbon potential of Hith- Gotnia evaporitic reservoirs, which consist of typical evaporites with high pore pressures. The well data acquisition is limited and very sparse. However, the deeper-drilled wells penetrate the Hith-Gotnia section and traditionally are isolated by casings to mitigate drilling hazards. Despite the well data gaps, there is excellent countrywide 3D and 2D seismic coverage. To continue the exploration endeavor in these challenging reservoirs, we designed an innovative and intuitive workflow to address seismic attribute analysis. The main objective of the study is to improve the understanding on what to target within Hith-Gotnia formations and where to find potential hydrocarbon accumulation to aid in successful drilling.

The Sabatayn Basin is located in Onshore Central Yemen and is part of a triple-junction system of grabens which developed in response to the break-up of Gondwanaland and opening of the Indian Ocean during the late Jurassic. The Sabatayn Basin is distinguished from the other rift basins of Yemen, such as the Masila Basin located immediately to the northeast, by the deposition of the thick late to post-rift Jurassic salt deposits of the Safer Formation. The presence of this salt is a critical element in the success of the prolific Alif play. Firstly it provides an effective top-seal for the Alif sandstone reservoirs which are interbedded within the basal section of the Safer Formation. In addition, salt mobilization during the Cretaceous was responsible for the formation of roll-over traps such as the Halewah Field. In some instances fractured basement seems to act as a migration conduit between the Jurassic Lam and Meem source rocks and Alif play traps. The presence of salt has a downside as it is responsible, along with irregular topography, for the poor seismic imaging of sub-salt exploration targets thus making the definition of basement trap geometries highly challenging.

We present the petrophysical joint inversion (PJI) of seismic and EM attributes to estimate the petrophysical model in terms of porosity and water saturation. This study realizes the joint inversion within the probabilistic structure provided by the Bayesian theory. The algorithm is applied to a real hydrocarbon exploration scenario to evaluate its contribution to the reservoir characterization. 3D volumes of estimated porosity and saturation show how the joint inversion of acoustic impedance and electrical resistivity can provide a quantitative description of the reservoir properties together with a measure of uncertainty.

New technologies in borehole and surface-seismic data acquisition and processing have allowed the integration of borehole information, such as Walkaway, Zero Offset VSP’s and dipole sonic logs, with surface seismic data. The aim of this integration is to produce seismic images of improved resolution, which can subsequently be ready conditioned for inversion and reservoir characterization. This paper describes some of these new developments, and the results from a project in Minagish field, Kuwait.

Geological conditions for hydrocarbon potential have been identified in the Bornu Basin in north Eastern Nigeria. The basin is part of the Chad Basin and other tectonically related contiguous basins within the West and Central Africa Rift System (WCARS) formed from the failed extensional rifting at an aulacogen junction during the separation of the South American and the African continents. The Bornu Basin covers about one tenth of the Chad Basin with limited geological mapping campaigns and unsuccessful exploration for oil and gas. Significant gaps still remain particularly in the vast northern areas characterised by flat topography, lack of continuous bedrock outcrops with structures concealed beneath sand cover. Remote sensing and GIS techniques offer potential for improving structural geological mapping in the basin but has not been used for these purposes. This research presents the use of combined remote sensing data including Landsat ETM+, ASTER, SRTM and Radar with other existing geosciences data including gravity, seismic, aeromagnetic and well log data to constrain the structure and tectonics of the basin through lineament analysis. The research involved mapping of lineaments, lithology, palaeodrainage and palaeo-shorelines in the basin. Two physiographically distinct study areas are selected in the basin to provide an appropriate comprehensive coverage of the basin attributes. The study sub-area 1 having mostly flat topography and lacking surface outcrops but comprising all the data sets available provides an enabling ground control site while the study sub-area 2 where outcrops are present provides an enabling site for validation of inferred field mapping and new observations of unmapped geology in the basin.

China is rich in shale gas reservoir, and has bright prospect in this area; however, the terrain where the shale gas reservoir located is complicated and the formation is changeable, which makes the exploration difficult. Therefore , It is very essential to find out shale gas reservoir favorable area and conduct further research on the sweet prediction technology for the breakthrough of shale gas reservoir exploration in China. At present, the critical factors of shale gas sweet prediction are mainly TOC prediction and brittle prediction, which normally have close relations with rock physics analysis. In the study, we divide rock physics analysis into log curve correction and inversion, rock physics perturbation analysis and favorable elastic parameter crossplot. Through this way, to find out the relative sensitive parameters to the elastic parameter variation and TOC and brittleness properties; and to form TOC and brittleness horizontal distribution regular patterns with the seismic inversion technology; finally to achieve shale gas favorable area prediction by combining seismic and logging with synthetic gathers and AVA analysis. With the actual shale gas exploration and research data in south China, we have a preliminary analysis and study on the status of rock physics technology application on shale gas sweet prediction, and provide a train of thought for the related study in this field. Through our research, we, for the first time, apply effectively the shale gas log curve correction and inversion prediction, rock physics perturbation analysis, favorable elastic parameter crossplot technologies to the actual shale gas exploration in south of China and form effective research methods. The research is significant in the way of providing important reference basis for shale gas reservoir sweet prediction.

This article is reporting the implementation of integrated energy constrained stochastic inversion to Group F Malay Basin reservoir. The technique involved Bayesian seismic inversion which use frequency domain seismic attribute as a constraint instead of time domain seismic amplitude. Maximum Amplitude Weighed Integrated Energy Spectra is proposed energy spectral attribute to be used to constrain the stochastic process. The new attribute is obtained by multiplying integrated energy spectra with maximum amplitude of accordingly seismic trace. It is found that Maximum Amplitude Weighed Integrated Energy Spectra provides a more separable signature in responding bed thickness changes than time domain seismic signature. Therefore, Maximum Amplitude Weighed Integrated Energy Spectra is supposed to be better in detecting seismically thin-bed structure. Its low degree of ambiguity in sensing thin-bed seismic is a potential to increase the vertical resolution and reduce of thin bed interpretation uncertainty. This article incorporates the maximum amplitude weighed integrated energy spectra with a Bayesian seismic inversion by using it to control the degree of match (likelihood) between seismic model and data. It insures that thin-bed structure can be accessed and sufficient resolution result can be attained. An inversion of near and far stack of a Malay Basin's seismic confirm that AVO attribute is better mapped after the seismic are inverted to reflection coefficient or relative acoustic impedance volume rather than from original amplitude. The gas sand penetrated by an appraisal well is detected by reflectivity coefficient AVO cross-plot while it does not appear in the same cross-plot from amplitude.

The Sivas Basin in Anatolia is likely the world’s finest open-air museum of salt tectonic structures. It is an elongated Oligo-Miocene sag basin that developed in an orogenic context above the complex Taurus-Pontides suture. A mid Oligocene quiet period in an overall continuous convergence history allowed the deposition of a thick evaporite sequence. Erosion of the Taurides shed clastic sediments and initiated the development of mini-basins and associated evaporite diapirs and walls. The minibasins are filled by Mid-Oligocene to Early Miocene clastics (fluvial silts and sandstones), marls, and lacustrine to marine limestones, the thickness of which may reach 4 kilometres. The stratal architecture along evaporite walls records the progressive subsidence of the minibasins, with strong rotation of beds, unconformities and local reworking of evaporites. Within the basin, the sediments show lateral thickness variations and spectacular angular unconformities. Following this quiet period, compression resumed in Early Miocene, forcing evaporites upward, which led to the formation of overhangs and sheets.