Case Study: Advanced Field-Scale Modeling of Near-Wellbore Data Resolves Complex Overthrust Structure

Extending conventional analysis of borehole image logs and dips to reservoir-scale modeling clears up uncertain seismic interpretation

Challenge: Determine location and geometry of main thrust fault, reservoirs, secondary faults, and natural fractures in a highly deformed anticline with poor seismic data and limited well control.

Solution: Retained Schlumberger DCS experts to analyze FMI formation microimager logs from two exploratory wells and use eXpandBG borehole geology software to perform field-scale structural modeling.

Result: Determined precise structural relationships between faults, fractures, and target reservoirs, enabling the client to obtain an excellent match with seismic data.

Unraveling complicated structural relationships

In 2008, an independent oil and gas company based in Australia became the operator of a promising, but underexplored, onshore block in Southeast Asia. During 2010, the company drilled two exploratory wells to investigate a sequence of deltaic-marine sandstone reservoirs within a complex anticlinal structure, deformed by thrust faulting, associated folding, and fracturing.

Owing to low-quality 3D seismic data and limited well control, the operator’s geoscientists differed in how to properly interpret the structural configuration of faults and reservoirs. As a result, the existing seismic interpretation had substantial uncertainties. Key questions included: Where exactly is the main thrust fault, and what is its strike and dip? Where are the secondary faults? What is the character of the natural fracture network?

To obtain reliable structural dips and improve its understanding of this highly complicated structure, the company decided to have Schlumberger run and interpret FMI fullbore formation microimager logs in the first of the two wells. High-definition FMI data was collected in this challenging freshwater environment.

Going beyond conventional FMI log analysis

The client engaged Schlumberger Wireline to acquire the FMI image logs and Data & Consulting Services (DCS) to perform a conventional near-wellbore analysis of the data in September 2010.

In addition to conducting the requested analysis, the DCS geology team in Kuala Lumpur went a step beyond the typical deliverable. Without referring to the seismic data, the team performed an advanced field-scale structural analysis of the FMI data and provided the operator a possible 3D schematic model of the structure’s complex geometry. To expand the near-wellbore analysis to reservoir scale, DCS used its eXpandBG borehole geology modeling technology, a new plug-in to the Petrel reservoir modeling system.

Results of the analysis were so well received that the company asked Schlumberger to interpret the same high-definition borehole image logs in the next well—also in a challenging freshwater environment—a month later. That well, which targeted potential reservoir sands in a separate fault compartment, encountered unexpected pressure anomalies in one section. When the near-wellbore analysis was complete, DCS integrated the FMI log and dip data from both wells and then elaborated and validated the 3D structural configuration. The entire FMI analysis took a little more than a week.

Achieving a match between model and seismic

The final FMI interpretation and field-scale structural modeling determined the precise thrust and fold geometry of the anticline, revealed the structural position and attributes of the reservoir units, computed true stratigraphic thicknesses, and provided a clear picture of the natural fracture network. Two fracture types were identified: Some were fold-related, but most were fault-related conductive fractures. Those erratic pressure points were explained by a brecciated zone above the thrust fault.

Not only was the final structural model consistent with regional outcrop studies, but also it enabled the operator’s geologists to successfully reinterpret the main thrust and secondary faults in the seismic data—yielding an excellent match between the model and the seismic.

Because the two exploratory wells encountered hydrocarbons in both shallow and deeper reservoir zones, the company plans to use its enhanced understanding of this complex structure to drill additional wells in the near future.

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Field, Reservoir, and Structural Modeling

Analysis of FMI image logs by the Schlumberger DCS Geology team Image data from the two wells (left), DCS consultants built the main thrust fault (center) using eXpandBG borehole geology softwareMain thrust fault
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"The robust well analyses and interpretations performed by Schlumberger opened up a new picture of the area, which was different than our previous understanding. This will serve as the basis for future logging and field-development planning. I’m pleased with the high quality of work Schlumberger delivered."
Marzuki Bin Mohammad Daud
Chief Geologist
Tap Energy (Borneo) Pty Ltd