Can a CSEM-supported exploration program increase the chance of success for a prospect portfolio by distinguishing between low and high saturation commercial hydrocarbons?
When defining an exploration strategy, the following points need to be considered:
Does the acreage contain several seismic amplitude-supported prospects and objectives?
Have false positive seismic DHI wells been drilled in the basin?
Is the seal affected by faults?
Does the seismic suffer from acoustic wipeout, suggesting fluid escape or gas attenuation?
How can associated subsurface risk be managed and mitigated?
This article addresses these issues and explains how a CSEM-supported exploration program increases the chance of success by distinguishing low saturation accumulations from commercial volumes and assessing the top seal potential.
(a) Seismic amplitude map reveals the presence of a double flat spot highlighted by the red and green lines. Faults are present at the top of the structure, suggesting possible leakage points.
(b) Interval average map of resistivity associated with the target level. Anomalous resistivity only coincides with the upper flat spot.
(c) Plot for water saturation (Sw) vs. seismic velocity (Vp) and resistivity (Rt). Vp is very sensitive to the presence of hydrocarbons in the system but cannot distinguish high vs. low Sw. Resistivity, however, is sensitive to low Sw.
(d) The structure was drilled twice: well 1 at the crest encountered high gas saturations and high resistivity; well 2 down dip encountered low gas saturations (< 7%) and no resistive anomaly. The integration of information allowed the operator to discriminate high vs. low saturation prospects. Furthermore, it suggested that the faults at the crest are leakage points. (Data courtesy of CNH/Pemex.)
Why is the Commercial Success Rate for Oil and Gas Production So Low?
Exploration statistics remind us of the poor commercial success rate, standing at about 12% worldwide (Westwood Global Energy, 2019).
Diving deeper into well results, it has been noted that a major cause of exploration failures is poor understanding
of seal and charge. Numerous dry or sub-economical wells have been drilled based on seismic direct hydrocarbon indicators (DHIs), which fail to distinguish between different reservoir fluid content. Low saturation fizz gas and high saturation gas accumulations typically have an identical anomalous seismic amplitude and amplitude variation with offset (AVO) expression. This ambiguity results in frustrating exploration campaigns where false seismic DHI plays have been drilled over and over again. This problem is well recognized by the industry (Wojcek et al., 2016; Cross et al., 2008) and it calls for a solution to obtain additional information about reservoir fluids to further de-risk exploration elements.
The industry’s efforts to improve the understanding of reservoir fluids have mainly focused on advanced seismic evaluation techniques, such as multi-component seismic, pre-stack processing and ocean-bottom cable seismic acquiring s-waves, all with the aim of unraveling the daunting fizz gas problem. However, even with these approaches, the issue of distinguishing fizz gas from commercial hydrocarbon accumulations still haunts the exploration outcome in many basins around the world. In other words, the value of obtaining additional seismic information may be very limited in view of the saturation uncertainty.
Integrating Technologies Brings Best Results for Hydrocarbon Exploration
If the industry wants to address the ongoing problem of hydrocarbon saturation and increase the exploration success rate, a subsurface measurement independent from seismic should be integrated with the seismic observations. Resistivity from CSEM is a natural choice for this, as saturation is the primary driver for the resistivity of a good quality reservoir with considerable volumes in place. Moreover, as shown in the figure below left, the saturation dependence of reservoir resistivity exhibits the opposite behavior to seismic reservoir properties and allows us to distinguish low from high hydrocarbon saturation.
Recent exploration activity has reignited interest in deepwater settings with a renewed focus on Tertiary siliciclastic turbidite environments. An exciting number of amplitude-supported leads and prospects have been identified from seismic in various offshore frontier basins, including Trinidad and Tobago, Colombia, Argentina and the Gulf of Mexico. In all these basins, explorers face the ongoing challenge of differentiating between high and low saturations from the seismic expression.
Close to ten years of experience of using CSEM for the evaluation of Tertiary turbidite plays in the Gulf of Mexico has shown that integration of resistivity with seismic information is the technology solution that is most capable of distinguishing between low saturations and commercial volumes of hydrocarbons. CSEM also provides an enhanced understanding of the reservoir properties that give rise to the seismic DHIs. The power of resistivity is illustrated by the case example in the figure (Escalera et al., 2014). Additionally, resistivity will provide an independent earth measurement of the rock properties associated with the top and lateral seal, allowing an explorer to characterize the rock properties of fluid escape features.
These results from the Gulf of Mexico demonstrate that CSEM can reduce the subsurface risk associated with seismic amplitude-supported prospects, high grading those prospects that are the best drilling targets or, conversely, identifying prospects that should be downgraded.
Returning to our initial list of questions, a key question that is missing may therefore be: has CSEM been considered in your exploration strategy? The integration of CSEM in an exploration program will allow an explorer to de-risk seismic amplitude-supported prospects, differentiating those that contain economic volumes and high saturations from shows or low saturation.
References:
Mavko, G., T. Mukerji, and J. Dvorkin, 2009, The rock physics handbook: Tools for seismic analysis of porous media, 2nd ed.: Cambridge University Press.
Schlumberger Wirelines Services, 1998, Log interpretation principles/applications: Schlumberger Educational Services.
Further Reading on CSEM in Hydrocarbon Exploration
A Strong Start to CSEM Services in 2019
GEO ExPro
EMGS has to date acquired 950 surveys in 36 countries and territories, from Spitsbergen to New Zealand, all calling on their specialist CSEM services.
This article appeared in Vol. 16, No. 3 – 2019
CSEM Anomalies Indicate Hydrocarbon Potential in the North Sea
Aslak Myklebostad and Svein Ellingsrud, PetroMarker
Vertical CSEM technology identifies new potential for undiscovered volumes around the Gjøa field in the North Sea.
This article appeared in Vol. 16, No. 4 – 2019
A New Approach to Hydrocarbon Exploration
Aslak Myklebostad; PetroMarker
PetroMarker’s unique 3D vertical EM technology acquires robust data for new development opportunities in already-explored areas such as the Norwegian Sea.
This article appeared in Vol. 15, No. 3 – 2018
