Sedimentology and sequence stratigraphy of the thistle field, northern north sea

Abstract

The Thistle field is located in the Brent Province near the northern edge of the Brent delta. Oil is contained within Middle Jurassic sandstones that can be divided into the five lithostratigraphic divisions of the Brent Group. First oil was produced in early 1978 and 355 mmbbl had been produced by end 1990. At that time significant oil was considered to remain in the Rannoch Sandstone Member of the Rannoch Formation, and a project was initiated to re-evaluate on-field core and wireline logs in a sequence stratigraphic framework. The aims were two-fold, primarily to test the reservoir model, but also to aid our regional understanding of the Brent system. At a reservoir scale, sequence stratigraphic analysis rapidly and independently produced a reservoir zonation closely comparable to the existing scheme. In addition, two new features emerged. Firstly, that the Rannoch Sandstone Member cannot be divided using sequence stratigraphie principles. Secondly, that a number of parasequences can be correlated field-wide within the Ness Formation. These parasequences are arranged in two parasequence sets, separated by a maximum flooding surface. Of likely regional significance is the recognition of four sequence boundaries within the Brent Group. The first, at the base of the Broom Formation, is inferred from the sharp erosional contact at base of the formation and the presence of coarse quartz grains within a thin transgressive systems tract that overlies the contact. A sandy lowstand systems tract basinwards of the Thistle field is predicted. The second sequence boundary is subtle. It is expressed as a field-wide increase in sand content at the base of the Rannoch Sandstone Member. This sequence boundary is close to its correlative conformity and has little down-dip potential. Three features indicate the presence of a third sequence boundary at the base of the Etive Formation: an erosive contact, a dramatic increase in grain-size and a downward shift in facies. The boundary is likely to be regionally significant down-dip. A fourth sequence boundary occurs at the base of the Tarbert Formation. Its down-dip significance is difficult to assess but may be low.