Fourth EAGE Exploration Workshop

There has always been continuous geophysical technological advances on multiple fronts, both on equipment (e.g. computing), the way of working (e.g. night operations), and processing algorithms. In PDO’s experience the major catalysts for change are twofold – collaboration with vendors, and motivated geophysicists. These have allowed us to experiment with different techniques without fear of failure.

The aggregate of these developments now enables dramatically increased channel counts, which in turn are funded by a step-change in productivity, for the requisite capex.

The ultimate result has been the potential for acquisition of properly sampled data in both source and receiver domains. Wide azimuth acquisition with adequate in-line and cross-line offsets, together with previously unthinkable fold are providing a new era of imaging. Further, coverage over very large areas with fast turn-around is nowadays a reality.

Reliable interpretation relies on accurate imaging and we can decline this one step further by saying that accurate imaging relies on an accurate velocity model. In the case of depth imaging and with the presence of salt, the velocity model itself should honour the velocity contrasts present at the Top/Base of the salt body. Here we present the application of a Salt Shape Tomography method on two datasets from the Sultanate of Oman. This method, which is part of multi-layer non-linear tomography, aims at reducing the time spent on manual interpretation or scenario testing for salt delineation, while producing better focusing for an improved subsalt image.

Significant improvements can be made to the imaging of shallow targets on land, broadband WAZ surveys when some care and attention is given to the processing sequence. A combination of better velocities, multiple attenuation, 5D interpolation and spectral shaping has produced superior imaging in the shallow section, leading to improved interpretation.

Near surface geology in south of Oman is believed to cause static instability, hence affecting seismic imaging. Such phenomena like low-velocity anomalies are suspected to generate pull-down artefacts as these features often correspond to intra-field lows. This effect is extends to deeper horizons and gets wider with depth.

Refraction static is usually used by seismic industry to construct near surface velocity. This has been proven successful in some areas. However, because it is data dependent and the method assumes an increase in velocity with depth, it will not work to identify low-velocity anomalies, suspected in this area. Therefore, alternative methods need to be investigated.

VSP, logs, Time Domain electromagnetic (TDEM), gravity lines, and 2D seismic were acquired. Each method is expected to give its own overview of the problem, but integrating all data will provide better understanding on extent and magnitude of the phenomena.

Resistivity logs converted to velocity for the upholes shows good agreement with sonic, which proved the robustness of Faust’s equation at study area, as results this equation enabled the construction of velocity model for near surface from TDEM data.

All data invalidate the existence of shallow anomaly. Therefore, the intra-field lows are considered to be real.

The Lekhwair formation is a new play for PDO discovered by serendipity in 2014 after encountering an oil bearing reservoir while drilling a Shuaiba well (L1). The well found oil saturations in the thin formation below Shuaiba (in the Lekhwair). Reservoir was not expected in this formation, originally interpreted as a unit of tight lagoonal limestones. Further, oil was flowed and sampled from the reservoir in an MDT run. A follow up dedicated exploration well, well L2, was drilled in 2015 to prove reservoir productivity and it confirmed a high quality and productive reservoir. The producing unit is only ~8m thick which is clearly below the seismic resolution of ~28m. Nevertheless, the reservoir stands out acoustically from the background tighter units by having significantly lower acoustic impedance (AI) properties. This enabled testing stochastic inversion to de-risk reservoir presence and continuity. Moreover, for the first time PDO used stochastic inversion outputs to calculate the STOIIP of the discovery to support statistical volume estimations.

Hidden Markov Model has been applied to predict the reservoir lithologies by using seismic inversion results as inputs. This method can take the conditional probability between different states or lithologies into account which is the vertical correlation in geology. In order to consider the misfit between the inversion results and the true well-logging data, the model needs to be trained. The application on a field example is quite successful in which most of lithologies have been predicted correctly even for some thin layers. However, this method is only 1D which means that the lateral continuity has not been considered yet.

The large reduction of oil prices has meant a focus of drilling of exploration wells to mainly low UTC, less risky plays. Reducing subsurface risks entails delivery of a robust well and seismic based framework to deliver good quality leads. Quantitative interpretation (QI), which has proven its value in prospect maturation, can also help at a play level evaluation.

This paper highlights the business impact of utilizing QI at an initial stage of prospecting in exploration.

Prospect evaluation identified structural and stratigraphic traps in shallow Early-Mid Cretaceous reservoirs in the Kauther area. Some of these demonstrated DHI’s (flat spots) in the seismic reflectivity dataset.

Seismic Inversion products support the presence of both good quality reservoir and hydrocarbons.

Intensive exploration efforts during the past three decades have resulted in discovery of many Jurassic oil structures in Kuwait. Once discarded as non-commercial, oil and associated gas accumulations in the Jurassic carbonate reservoirs are found to be significant and sustainable. During the journey of success we have learnt a few lessons and we are likely to learn many more in future.

Among the Jurassic plays the Marrat Formation is considerably thick and the reservoirs are highly heterogeneous, both vertically and laterally. On the other hand, the Mid to Late Jurassic Sargelu and Najmah Limestone are fairly homogeneous, but with very low porosity. Fractures are vital for the deliverability from these tight reservoirs. The overlying Gotnia and Hith evaporate cycles have yet another challenge, namely, stringers of carbonate reservoirs sandwiched between thin anhydrite layers.

In order to address the above concerns we have been regularly using seismic inversion as a part of our G & G interpretation workflow. Based on the problem definition, rock physics modelling and quality of seismic data, we have adopted different inversion workflows, both in post stack and prestack seismic data. The experience is effectively used in the identification of new plays and prospects in the least explored areas and formations. Some of the case histories shall be presented and discussed to illustrate the use of seismic inversion techniques in the context of Jurassic exploration in Kuwait.

The Development Leases operated by Shell Egypt in the Abu El-Gharadig Basin are covered by six 3D seismic, cross-spread, orthogonal seismic surveys acquired during the period from 2002 to 2008. The acquisition parameters for those surveys generated low to medium fold datasets with critical geophysical limitations impacting the overall data quality.

All of these seismic surveys were processed using a basic shot processing and Pre-stack Time Migration (PSTM) workflows at the time of acquisition.

In order to maximize the value of these legacy surveys a mega in-house re-processing project was kicked off to re-process the six seismic datasets covering all of the development leases in the basin. The objective of the re-processing was to deploy the latest processing technology on these surveys with acquisition limitations.

Results of seismic survey and data processing were considered as pattern of acquiring data in high folded structure area. Advantages of applying summation technology for the first Fresnel zone were showed in a comparison with traditional approaches. To solve the problem of identifying the nature of out of the plane reflections was showed results of ray modeling. Eventually there were offered alternative design of seismic acquisition and modern approaches to seismic data processing.