INTRODUCTION

3. STRATIGRAPHY AND RESERVOIR ZONATION

Lithostratigraphy versus sequence stratigraphy

The difference between lithostratigraphy and sequence stratigraphy is well illustrated on Figure 3.1. The dashed lines, some of them with circled numbers, represent time-equivalent lines through the stratigraphy, ie. each line touches sediments that are of equal age, and hence were deposited at the same time. These timelines represent, or are parallel to, levels in the stratigraphy where shifts in progradation or retrogradation take place. In the marine environment, progradation is roughly synonymous with regression, and retrogradation with transgression. Sequence stratigraphy is the science that stands behind the interpretation of these depositional trends, and behind the identification of surfaces that bound these trends. Most oil companies work with two different surfaces: 1) sequence boundaries, which separate progradation from retrogradation, and maximum flooding surface, which separate retrogradation from progradation (ie. are surfaces of maximum transgression, in opposition to sequence boundaries which are surfaces of maximum regression / retrogradation). Under special circumstances of high sediment supply, the sequence boundary may occur prior to (underneath) the turnaround from progradation to retrogradation (compare Sequence Boundary with Maximum Regression Surface in the lower part of the diagram in Figure 3.2). We will not get into the details of this, other then mentioning that such situations occur when there is erosion associated with the final stages of progradation, and that this erosion stops prior to the time when the whole system starts retrograding. The sequence boundary, in the Statoil terminology and also Exxon-Mobil (and likely several other companies), is set at the level of erosion, and not the level of maximum progradation / regression.

In contrast to sequence stratigraphy, lithostratigraphy has nothing to do with an interpretation of trends or depositional processes. It is simply the science that subdivides a sedimentary rock package into its most obvious lithologies (rock type). As seen in Figure 3.1, the main lithological units and their boundaries cross-cut (are at an angle to) the sequence stratigraphic surfaces / timelines. These main lithological units, as in the case of the Brent delta, are given group names, formation names, and/or member names.

Figure 3.2 shows the sequence stratigraphic surfaces and systems tracts (packages) that Statoil uses in reservoir and exploration geology.

Figure 3.3 is an outcrop analog to the Brent delta, namely the Mesa Verde Group of the San Juan Basin (southwest USA). Note also here the contrast between lithostratigraphy and sequence stratigraphy. This is especially the case for the prodelta / delta front area, ie. the Point Lookout Formation and the Mancos Shale. These units are roughly analogous to the Etive and Rannoch formations of the Brent Group.

Exercise #4 (Fig. 3.4)

Identify the main lithostratigraphic units on the correlation panel. That is: Broom, Rannoch 1, Rannoch 2, and Etive. The Broom is very thin, and has low gamma ray. The boundary between Rannoch 1 and Rannoch 2 is where the main lithology passes from poor quality sandstones and siltstones/shales, to good quality sandstones. Rannoch 1 has mainly a shale signature on the neutron-density logs (called positive separation), whereas the Rannoch 2 has mainly a sand signature (negative separation), or superposed log curves with no separation. As for the Etive, it has excellent reservoir properties.

Exercise #5 (Fig. 3.5)

Try to find stratigraphic sequences and correlate them from well to well. The sequences should be bound by sequences boundaries, that is lines of maximum progradation (levels of low gamma ray value, prior to a retrogradation / transgression). According to the model for the Brent delta, the sequences boundaries should be oblique and climb down the stratigraphy from left to right (north to south). But at the scale of the Statfjord Field, how oblique (or straight) are really these timelines ? It is for you to find out..Solution is on Figure 3.6.

Examples of reservoir zonations

Reservoir zonations on most fields are usually based on sequence stratigraphy correlations. But since the principles of sequence stratigraphy were only established at the end of the 1970s, fields that are relatively old, like the Statfjord Field, first operated with a lithostratigraphic zonation. This lithostratigraphic zonation on the Statfjord Field is called LITHO_Statfjord (Fig. 3.7). It is still useful and actively in use, because locally the formation and zone boundaries (ie. Rannoch 1 and 2) have petrophysical contrasts that make the units distinct flow units.

But at a smaller stratigraphic scale, one needs a sequence stratigraphic zonation (called SEQUENCE_Statfjord on the Statfjord Field). For example, the sequences you have delineated in the Rannoch Formation are important flow units, even if at the large scale they cross-cut the lithological units (ie. Rannoch 1 / Rannoch 2). In the case of the Ness and Tarbert SEQUENCE_Statfjord zonation, the individual zones have boundaries that are parallel to the lithostratigraphic boundaries, and so it is simpler to identify the main flow units (ie. Fig. 3.8, Tarbert Fm). This is also largely the case for the zones in the Statfjord Formation.

Figure 3.9 shows an example of reservoir zonation from the Hugin Formation of the Sleipner Field. The Hugin is time equivalent with the Tarbert Formation, and occurs in the southern part of the Brent delta (see Fig. 3.1). One sees on close observation that the Hugin zonation follows sequence stratigraphic boundaries. In some places on the field these surfaces are parallel / coincide with the main lithostratigraphic changes (south and north on the field), while at other places there is a significant difference (central on the field, where the upper shoreface / barrier sandstones pass northwards into offshore shales).

Exercise #6 (Fig. 3.10)

This three-well correlation panel is from the Raude Member of the Statfjord Formation, taken from the central part (A platform) of the field. Based on the sequence stratigraphic representation on Figure 2.1, identify the reservoir zonation that is utilized on the Statfjord Field for effectively draining the Raude Member. There are five zones in total. Each zone should be relatively sand-rich in the lower part, and variably shale-rich in the upper part. The solution is shown in Figure 3.11.

Through these exercises in stratigraphy, we have come across three types of sequences in terms of log pattern. The three types we encountered are (see Fig. 3.12) : 1) small-scale bay-head delta sequences in the Ness, which show coarsening-upward trends on the gamma ray log (other patterns are also present in the Ness zones) ; 2) shoreface coarsening-upward sequences in the Rannoch Formation; and 3) alluvial (river) fining-upward sequences in the Raude Member of the Statfjord Formation. All these types are flow units, and are retrograding-prograding, or transgressive-regressive, sequences.