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?

 

Figures 

	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.

 

Figures 

	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).

 

Figures 

	Logging interpretation validates the variance indications.
	Variance indications of different types of reservoir.
	Variance abnormalities indicate bottom water.
	Fractured carbonate reservoir model.
	Caves and vugs can be identified on logging data.
	Field reservoir properties statistics: Tight fractured carbonate.
	Karst layers validated by formation images.
	Production testing statistics: Vadose and Phreatic are major net pay zones.
	Variance abnormalities calibrated by logging data.
	Real story: Before sidetrack no production; after sidetrack (according to variance abnormalities) a good producer.

 

Conclusion 

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.