Fault Influence on Fluid Flows and Reservoir Diagenesis

Mingchou Lee (ExxonMobil, deceased), 
Jurgen Lewandowski (formerly Mobil Erdgas-Erdol Gmbh, Celle, Germany),
Klaus Kronmuller (ExxonMobil, formerly Mobil Erdgas-Erdol Gmbh, Celle, Germany)

An integrated methodology for studying reservoir heterogeneity in complex geological settings influenced by facies, structure and diagenesis has been derived for Upper Rotliegend (Lower Permian) sandstones in the NW German Basin. The methodogy derived is iterative. First, a preliminary geologic model was constructed based on logs, facies description, petrography and core porosity and permeability. Second, a set of core samples were selected to evaluate timing, paragenesis, and the spatial distribution of diagenetic processes. This approach allowed us to critically evaluate the validity of the model and quickly modify the model when deviation from the predicted outcome was found. 

In this basin, the main producing sandstones are the Schneverdingen, the Dethlingen, and the Wustrow. Although the Schneverdingen is mostly located in the water zone, it has the best reservoir quality and is the best producer in the field. The Wustrow and Dethlingen sands are more heterogeneous and could not be correlated by facies alone.

The reservoir quality study revealed that detrital grains of the Wustrow and the Dethlingen were altered more extensively in samples near the EW trending, fractured/faulted transfer zones than in samples near the graben center. Four alteration zones are recognized. In these alteration zones, cement compositions appear to gradually change. Integrated evidence show the effect of acidic, highly evolved, and CaCl₂ - rich saline fluids during cementation at temperatures around 140 to 170^oC. During two separate episodes which peaked at around 205 and 180 ma, diagenetic fluids entered the reservoir through fractured fault-transfer zones, and altered nearby Wustrow and Dethlingen sandstones. Away from the entry zones, sandstones in the paleo-HC zones were protected from further alteration. The Schneverdingen suffered only limited alteration due to the fact that the sands were sealed from lateral fluid movement by tight conglomerates, which extended into the fluid entry zone. Timing of the two events can be correlated to active rifting both sides of the Atlantic Ocean and in NW Europe, when hydrothermal fluids were active.

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