Sedimentological Correlation of Heterogeneous Reservoir Rocks: Effects of Lithology, Differential Compaction and Diagenetic Processes

Abstract

Correlation of reservoir rocks is a challenge in many types of reservoirs around the world today. In a time where the focus has slowly shifted away from the giant fields, towards development of smaller, more marginal fields, the importance of addressing these challenges is increasing. The purpose of the thesis is to investigate the effects of lithology, diagenesis and differential compaction on correlation of reservoir architecture in a fluviodeltaic reservoir. To be able to address these issues several types of data, including seismic, core sections and petrophysical wireline logs, have been subjected to measurements, calculations and interpretations to be able to construct a sedimentological and sequence stratigraphic framework based on a reservoir reconstruction workflow.

First, the regional and local (field wide) geological evolution for the Åre Formation in the Heidrun Field, offshore Mid-Norway, is elucidated, including evolution of the palaeodepositional environment and the prevailing depositional controlling factors. These studies included a facies description of studied wells based on core and wireline log data, and description/ interpretation of the reservoir sequence stratigraphy. From these studies eight facies associations have been identified and described, indicating a fluvial-deltaic depositional environment. A sequence stratigraphic model based on these facies associations is suggested for the studied well data and includes five candidate sequence boundaries and eight flooding surfaces, including a marine flooding surface. Four of these surfaces are interpreted as allogenic, suggesting regional base level change, whereas the remaining nine surfaces are suggested as resulting from local, autogenic factors.

Next, the compactability of the identified facies associations was quantified in a diage nesis study using optical microscopy, SEM micro-probe measurements and XRD analyses. The paragenetic sequence proposed suggests that abundant early siderite cement has influenced the compactability of fine-grained deltaic siltstones. Eogenetic meteoric flushing led to leaching of detrital feldspar and precipitation of authigenic kaolinite in the fluvial part of the succession. Other cements include calcite, ankerite and Fe-dolomite which are interpreted as predominantly mesogenetic and of less importance regarding compaction. However, a persistent calcite cemented interval is suggested to be related to a flooding regional event and may therefore have sequence stratigraphic significance.

In a reservoir reconstruction study, data on lithofacies compactability and sequence stratigraphic surfaces (i.e. flooding surfaces) were combined to perform differential decompaction of the reservoir in a sequential re-burial exercise. A methodology for reservoir reconstruction is proposed based on porosity reduction vs burial for identified lithofacies classes and the application of flooding surfaces as backstripping surfaces. Based on correlation on decompacted sediment cross-sections, new horizons are identified within the lower part of the Åre Fm. (Åre 1-3.3), an interval suggested to have been subjected to large differential compaction effects due to abundant coals. These surfaces includes several local flooding surfaces related to coals, in addition to laterally correlatable channel sandstone units. The method is flexible and performed for each depth interval (15cm resolution) throughout the well.

As the proposed method is applied on real wireline log data, some sources of error are related to the manual interpretation of lithofacies classes, constituting the building blocks for the proposed model. An attempt is therefore made to interpret within lithological heterogeneities based on multivariate statistical techniques. Unstructured and structured principal component analysis has been applied to five wireline log variables (GR, NPHI, RHOB, RT, DT). The results indicate a clear beneficial potential of improving the differentiation between different lithological and depositional units by extracting small scale heterogeneities