--> One Direct Indicator for Reservoir in Fractured Carbonate, by GuangJun Chen, Mohamed Redal, Jianguo Zhu, and Wei Meng, #40258 (2007).

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^PSOne Direct Indicator for Reservoir in Fractured Carbonate*

By

GuangJun Chen¹, Mohamed Redal¹, Jianguo Zhu², and Wei Meng²

Search and Discovery Article #40258 (2007)

Posted September 2, 2007

*Adapted from poster presentation at AAPG Annual Convention, Long Beach, California, April 104, 2007

¹Schlumberger, Beijing, China ([email protected])

²Northwest Bureau, SinoPec

The reservoirs in the Tahe oilfield are fractured carbonates with extremely severe anisotropy, typically karsted. The rock is a very tight limestone, with matrix porosity of < 2%. Vugs and caves connected by natural fractures are the storage. Reservoirs are developed along with a weathering crust. Some wells produce oil at a relative high rate, while others have a very quick breakthrough or produce water initially. Accurate characterization of the reservoirs to enable increased oil and decreased water production is a challenge.

In an attempt to directly characterize the reservoirs seismic variance was used, which calculates the direct measurement of dissimilarity of seismic traces and produces much sharper and more distinct terminations than those observed in normal amplitude data. Calibrated with core, log, and high-resolution resitivity data, fractures, caves, and vugs are all shown to result in high variance values, indicating that a reservoir can be developed only where variance shows high values. Variance results are displayed in 3D or in cross-sections, and are used to identify the geological features in the volume. Based on the variance indicators, caves and vugs can be directly identified to predict the carbonate reservoirs, which significantly aids properties modeling and production management.

While seismic variance does not provide a perfect definition of the reservoir, it has proven to be a reliable direct indicator for reservoir facies in the fractured carbonates of the Tahe field.

Background and Challenges

Structurally, the area is relatively simple, gently sloping from northeast to southwest.
The rock is a very tight limestone, with matrix porosity being < 2%.
The reservoir porosity developed as a result of long-term weathering and erosion, its storages are vugs and caves connected by natural fissures.
The Ordovician system is the main hydrocarbon carrier.

Reservoirs have extremely severe anisotropy.
Some wells produce oil at a relatively high rate while others have a very quick breakthrough or produce water initially.
How to characterize the reservoirs to enable increased oil and decreased water?

	Depth surface of target Ordovician.
	Evolution of the karst system.
	Production out of the karst system.
	Breccia and vug.
	High angle corrosion fracture.

Solution and Workflow

Used seismic variance process to calculate the direct measurement of dissimilarity of seismic data to highlight much sharper, more distinct terminations.
Combined geological analysis and log data interpretation to describe the fine differences related to faults, fractures, and strata information such as the bounding limits of a reservoir.

Reservoir style determination.
Seismic variance processing.
Abnormities calibration with logging data.
Abnormities validation with production testing.
Fractured reservoir identification and description.
Characterization output for modeling.

	Seismic variance processing, 20 ms and 5x5 result were used.
	Fractured variance abnormality validated by well and core.
	Vug variance abnormality with logging curves.
	Variance abnormalities, identification and characterization.

Indications

Reservoir can only develop at big variance abnormalities.
No variance abnormalities, no reservoir.
Standing or vertical variance abnormalities represent the existence of vertical fractures or small faults, which play an important role in reservoir connection.
Upper variance abnormalities produce oil while lower usually do so with water, if they are connected by vertical fractures (possibly with oil & water).

While seismic variance does not provide a perfect definition of the reservoir, it has proven to be a reliable direct indicator for reservoir facies in the fractured carbonates of the Tahe field.

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