North Cape Province
Forward-modeling Methodology Application to Reduce AVO Analysis Uncertainty Using Well And 3D Seismic Data From Ibhubesi Field, Orange River Basin, RSA
Spikes, Kyle (Stanford Rock Physics Laboratory, Stanford University) | Dvorkin, Jack (Stanford Rock Physics Laboratory, Stanford University) | Strecker, Uwe (Rock Solid Images)
ABSTRACT Interpreting bright spots on amplitude stack data and/or of amplitude versus offset data (AVO) in terms of lithology, fluid, and porosity (LFP) may lead to ambiguous conclusions resulting in the of drilling of a dry hole. Rock physics-based forward modeling of seismic data presents an avenue to resolve this ambiguity by relating controlled perturbations in the underlying rock properties to diagnostic changes in seismic signal character. The purpose of this work is to apply a geologic- and rock-physics-based forward-modeling methodology to a world-class gas reservoir, namely the AY-1 producing gas well of Ibhubesi Field, Orange River Basin. The forward-modeling process consists of five steps: (1) rock physics diagnostics and geologic interpretation of well and seismic data, (2) viable geologic model generation, (3) establishment of petrophysical transforms and population of the geologic models with realistic LFP values, (4) translation from rock properties to elastic properties, (5) synthetic seismogram generation and perturbation of the underlying rock properties to match the synthetic responses to the real seismic responses. Using well and seismic data from this sand-shale succession, we focus on the rock physics diagnostics, the geologic interpretation and seismic attribute analysis, and the statistical perturbation steps of the forward-modeling methodology. Seismic data provide the interpretation of the geologic setting supply information to predict reservoir properties away from well control.
- Africa > South Africa > North Cape Province (0.63)
- North America > United States > Texas > Orange County (0.61)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock (0.35)
- North America > United States > Wyoming > Sand Field (0.99)
- Africa > South Africa > Orange River Basin (0.99)
- Africa > South Africa > North Cape Province > Orange Basin > Block 2A > Ibhubesi Field (0.99)
- (2 more...)
Abstract: This paper will focus on the vast deepwater areas of offshore South Africa; areas which require a challenging and expensive exploration effort. Latest exploration activities of Offshore South Africa, combined with a bit of exploration history will be discussed. This would then be compared with worldwide trends concerning deepwater exploration. Africa's important role within this new drive to exploit the world's deepwater hydrocarbon resources is also emphasized. General geology of the offshore basins will briefly be summarized, and the talk will coclude with the latest resource estimates of South Africa's deepwater areas. INTRODUCTION During the past 30 years, exploration of the Mesozoic basins of offshore South Africa was generally restricted landwards of the 200m isobath, with only a few wells exploring the Orange Basin on the West Coast in water depths deeper than 400m. Off the South Coast these rigs could only manage in water depths of up to 300m maximum. The strong Agulhas current and adverse weather conditions imposed restrictions on the use of the existing semisubmersibles. Exploration history The first offshore well in 1968 discovered the Superior gas field in the Pletmos Basin, which is a sub-basin of the Outeniqua Basin off the South Coast. Subsequently, most of the offshore exploration was then focussed in the Bredasdorp Basin, which led to production of the F-A, EM and satellite gas fields in 1992, which have been feeding the synfuel plant at Mossel Bay. First oil production from the Oribi oilfield followed in May 1997, a deep marine basin floor channel and fan complex (bff complex), and production was increased when the Sable oil, gas and condensate field came onstream in mid-2003. The Orange Basin is defined by the extent of a sedimentary wedge that occupies about 150 000 km off the southwestern coast of Africa, and is more than 8 km thick in places. Although the basin is sparsely drilled with only 40 wells, most of which tested the shallow water areas, results thus far have been extremely encouraging. Two gas fields (Ibhubesi in South Africa, and Kudu in Namibia) with multi-TCF potential have been discovered within the younger geology, while the A-J1 well yielded an oil discovery in some of the oldest sedimentary fill. The Ibhubesi gas field has recently been defined by Forest Exploration International, through appraisal drilling in the area of the original A-K1 discovery well. The tested wells in this field yielded a high combined flow rate of dry gas and condensate. Analysis of 2D and 3D seismic surveys in the area has defined many new prospects and justifies the extension of this play for some distance to the north. The Durban Basin remains under-explored with only 4 wells testing the offshore areas close to shore, all of which were non-commercial. World trends Studies of oil discoveries made during the nineties indicate that giant oil discoveries are more frequently found in the deepwater areas of the world compared with the shallower water regions.
- Africa > South Africa > Indian Ocean (0.69)
- Africa > South Africa > North Cape Province (0.54)
- Geophysics > Seismic Surveying > Seismic Processing (0.94)
- Geophysics > Seismic Surveying > Surface Seismic Acquisition (0.69)
- Africa > South Africa > South Atlantic Ocean > Orange Basin (0.99)
- Africa > South Africa > Pletmos Basin (0.99)
- Africa > South Africa > Outeniqua Basin (0.99)
- (10 more...)
- Reservoir Description and Dynamics > Reservoir Characterization > Seismic processing and interpretation (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Exploration, development, structural geology (1.00)
- Management > Strategic Planning and Management > Exploration and appraisal strategies (1.00)
Abstract General Elastic Inversion is a specific inversion process that is designed to combine both ?Vp (pseudo compressional interval velocity) and ?Vs (pseudo shear interval velocity) volumes in a straightforward and meaningful way to emphasize pore fluid type. The calibrated results are simple and values nonarbitrary. Good data seismic quality and high fold are preferred, and a good quality dipole sonic curve from an area well can be used for calibration but is not required. In essence the process has 2 steps and proper execution of both is required to get good results. The first step is generation of the ?Vs and ?Vp volumes and the second step is the combination and re-projection of the ?Vp and ?Vs volumes. A critical step is generation of the ?Vs volume which must be done in the pre-stack domain. The objective is to estimate converted-wave shear amplitude at different offset angles. On problem is that some methods result in only a linear transform of the P-wave amplitudes which will not produce a separation of gas and water cases in the P and S crossplot domain. We present a rigorous 'full Zoeppritz' solution involving ray-tracing that provides meaningful, nonlinear calculation of what the converted shear-wave amplitudes by offset would have been had they been recorded. In essence, the S wave AVO is modeled. Secondly the P and S reflectivity volumes are inverted, scaled to pseudo-velocity, and then calibrated to well control. The resulting ?Vp and ?Vs volumes are combined and reprojected to produce a 'fluid' volume and a 'porosity' volume. The process is simple enough in concept that we propose it as a general solution to Elastic Inversion. Like the Fluid Factor (Smith and Gidlow, 1987) and Rp-Rs (Castagna and Smith, 1994) methods it combines both volumes by summation of Vp and Vs. Examples are from the giant Ibhubesi gas fields offshore the Republic of South Africa in the Orange River Basin,Fig. 1, Location of Ibhubesi Gas Field, offshore the West Coast of the Republic of South Africa. (Available in full paper) however this method has been used in other area, both land and marine, with excellent predictive results. Introduction to the Inverse Problem What is Inversion? The name implies it is an inverse method, that is-one that has multiple possible solutions. This is the opposite of the forward method, where the conditions and experiment are known and the results are measured and repeatable. The results are always the same and the experiment may be repeated. With the inverse method (and more specifically the 'Inversion' used here,), the results and experiment are known and we are asked to solve for the original conditions. While it is true that there are multiple solutions, that is different geologic conditions may give rise to the same measurement, usually the practical possibilities may be narrowed. Most geological problems are in fact inverse procedures and understanding this relationship is both simple and profound, as non-uniqueness imposed by the inverse method is the fundamental reason for risk and uncertainty in Geology and Geophysics.
- Africa > South Africa > North Cape Province (0.54)
- Africa > South Africa > South Atlantic Ocean (0.40)
- Africa > Namibia > South Atlantic Ocean (0.40)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock (0.69)
- Geology > Geological Subdiscipline (0.69)
- Europe > United Kingdom > North Sea > Central North Sea > Central Graben > Greater Stella Area > Block 30/6a > Stella Field > Stella Ekofisk Formation > A2 Well (0.99)
- Europe > United Kingdom > North Sea > Central North Sea > Central Graben > Greater Stella Area > Block 30/6a > Stella Field > Stella Andrew Formation > A2 Well (0.99)
- Europe > United Kingdom > North Sea > Central North Sea > Central Graben > Greater Stella Area > Block 30/6-2 > Stella Field > Stella Ekofisk Formation > A2 Well (0.99)
- (4 more...)