Abstract: Correlation Between Oil Families and Source Rocks in Northern Part of Timano Pechora

BJOROY, MALVIN, KJELL ARNE BAKKEN, and PETER B. HALL, Geolab Nor AS; VALERY RASSOMAKHIN, and OLGA TIMOCHENKO, Arkhangelskgeologia.

Introduction

A detailed petroleum geochemical study has been performed over the northern part of the Timano Pechora area involving analysis of 89 oils and 146 sediment (core) samples. Oil and gas exploration have been undertaken in the Timano-Pechora area since the mid-sixties and since then more than sixty oil and gas fields have been discovered. However, only minor petroleum geochemical information has previously been available. The areas studied are the Varandey-Adzvin zone, Khoreyver area and Pechora-Kolva aulacogen. Oil and gas reservoirs have been found in strata ranging from Lower Triassic to Lower Silurian.

Geology

The northern part of the Timano-Pechora can be divided into five separate parts. These are, going from west to east, the Malozemelso-Koguyev monocline, Pechora-Kolva aulacogen, Khoreyver depression, Varandey-Adzvin zone and Khorotaikhin depression. Only the middle three areas have any oil or gas potential. Our study has therefore been concentrated on these three areas.

Within the studied area the tectonic character of the sedimentary cover depends on structures and interactions between/within the Timano-Pechora platform and the Ural fold belt. Three major structures are distinguished: the Timano ridge, the Pechora syneclise and the pre-Ural marginal trough. Together with the foreland folds of the Urals they compose the Timano-Pechora platform.

A rift stage occurred during the Cambrian and Early Ordovician. During this period general uplift predominated over subsidence, so that active erosion of the post-orogenic consolidated Baikal platform took place. The erosion was accompanied by sedimentary infilling of rift valleys with continental red-coloured volcano-terrigenous deposits.

A syneclise stage occurred during the Early Ordovician to Early Permian (Artinskian stage). Active subsidence with marine transgressions caused accumulation of marine, terrigenous and carbonate formations Sediment thickness can reach 6.5 - 7 km. Sedimentation in Middle Ordovician to Lower Devonian was mostly marine shaly carbonate. These conditions are favourable for oil source rock deposition. The Middle Devonian experienced times of near shore/delta/deltafront deposition (sandy, coaly sediments). Thereafter more marine conditions returned. The Late Devonian experienced deposition of marine shales and shale-carbonate. These conditions are also favourable for oil source rock formation. During the Carboniferous there was a variable deposition of sand and carbonate.

A platform orogenic stage started in the later part of the Carboniferous/early part of Permian with erosion and terrigenous sedimentation. A regime with sedimentation and erosion followed. In the Middle Triassic to Early Cretaceous the formation of the area was mostly complete.

Oil families

The geochemical data separates the analysed oils into four main families. Some of these are separated into sub-families. Also, some of the oils were found to be mixtures of oil families.

Oil family 1, which consists of two sub-families, is found in post-Devonian reservoirs in the Pechora-Kolva area. These have maturities equivalent to 0.9 - 1.0 % Ro vitrinite reflectance, i.e. past peak oil generation. The two sub-families are separated by variations in the biomarker data. One of the sub-families occurs mainly in the Shapkina-Yoryaka swell, the Denisov depression and the Kolguyev Island. The other sub-family consists of the post-Devonian oils in Pechora-Kolva outside these areas. The difference between these sub-families are attributed to local variations in the source rock.

Oil family 2 is found in Devonian reservoirs in the Laya and Kolva megaswells of the Pechora Kolva area. These oils have maturities similar to those in family 1; equivalent to 0.9 - 1.0 % Ro vitrinite reflectance. This oil family is also, divided into two sub-families based on variations in their biomarkers. Again this is attributed to local variation in the source rock.

Neither family 1 or family 2 oils were biodegraded but some were found to be water washed. Both families consist of low-sulphur oils, with low nickel and vanadium contents. They have low densities with API gravity ranging 67° - 31°.

Oil family 3, which consists of two sub-families, is found in the Varandey-Adzvin Zone and Khoreyver area. The reservoir rocks range in age from Upper Devonian to Lower Triassic. This oil family has maturities ranging from 0.6 to 0.8 % Ro vitrinite reflectance equivalent, i.e. pre- to peak-oil generation maturity. This oil family has a far higher sulphur, nickel and vanadium content than oil families 1 and 2. They also have a higher density, with API gravity ranging from 33° to 11°. The two sub-families can be differentiated by their pristane/phytane ratios. This is yet again attributed to local variations in the source rock.

Oil family 4 is also found in the Varandey-Adzvin zone and Khoreyver area, but in reservoir rocks no younger than Devonian and mostly Lower Devonian and older. Some of the oils in the Varandey-Adzvin zone and in the Khoreyver area are mixtures of oil family 3 and 4.

Source Rocks

Source rock analysis was undertaken on a total of 146 core samples. The source rocks fall into four main groups, which correlates to the oil families mentioned above.

Source rock group 1 is found in the southeastern part of the Pechora Kolva area. It is of Devonian age. The maturity of the analysed samples is high, about 0.9 - 1.1 % Ro vitrinite reflectance. The maturity fits with that found for oil families 1 and 2.

Source rock group 2 is found in the remaining parts (northwestern) part of the Pechora Kolva area. This source rock group is also of Devonian age. The maturity is similar to that of group 1 source rocks, i.e. 0.9 - 1.1 % Ro vitrinite reflectance.

The above discussed source rock groups in the Pechora Kolva area are both liptinite rich and are estimated to have had (and still have in shallow occurrences) hydrogen indices of 500 - 600 mg HC/g TOC. The thickness of good source rocks of group 1 and 2 is estimated to be on the scale of 100 m. The good source rocks are interbedded with poorer source rocks. It estimated that these two source rock groups have generated and expelled more than 20 billion metric tons of oil and gas. This is enough to explain all the reservoired oil and gas in the area.

Source rock group 3 is found in the Varandey Adzvin zone and in the Khoreyver area. This source rock group is of Upper Devonian age. These source rocks have maturities ranging from 0.6 to 0.7 % Ro vitrinite reflectance. This is similar to the maturity found for oil family 3. Source rock group 3 has many similarities to source rock group 1 and 2 in the Pechora-Kolva area and are of the same age. However, group 3 samples have so many differences in hydrocarbon composition that they have been put into this separate group.

Source rock 4 is found in the Varandey Adzvin zone and in the Khoreyver area. This source rock group is of Lower Devonian to Silurian age. This source rock group is slightly more mature than source rock group 3 and the maturity fits well with oil family 4.

Both source rock groups 3 and 4 are estimated to have original hydrogen indices of 500 - 600 mg HC/g TOC. The thickness of good source rock is estimated to be on the scale of 100 m. Calculations indicate that these two source rock families have generated and expelled more than 1 billion metric tons of oil, which is enough to explain all the oil in the system. The reason for the lower oil yield of the source rock families in the Varandey Adzvin zone and in the Khoreyver area, compared to the Pechora Kolva Aulacogen is that the former source rocks have a lower maturity (they have mostly not reached peak oil generation).

Conclusions

Four oil families have been identified (some divided into subfamilies). Oil family 1 is found in post-Devonian reservoirs of the Pechora-Kolva aulacogen. Low sulphur and low density oils with high maturity. This oil family correlates with source rock group 1, which is of Devonian age and located in the southeastern pan of the Pechora-Kolva aulacogen.

Oil family 2 which is found in Devonian reservoirs in the Pechora-Kolva aulacogen consists of low density and low sulphur oils with high maturity. This oil family correlates with source rock group 2, which is of Devonian age mid located in the northwestern pan of the Pechora-Kolva aulacogen.

Oil family 3 is found in the Varandey-Adzvin zone and Khoreyver area in reservoirs of Upper Devonian to Lower Triassic age and consists of high sulphur and high density oils of low maturity. This oil family correlates with source rock group 3 which is found in the same areas and is of Upper Devonian age.

Oil family 4 is also found in the Varandey Adzvin zone and Khoreyver area in reservoirs of mostly Lower Devonian and older age. This family also has high sulphur and high density oils with low maturity. This oil family correlate with source rock group 4 which is of Lower Devonian and Silurian age. Both the oils and the source rocks of type 4 have slightly higher maturity than those of type 3 which fits well with their deeper stratigraphic occurrence.

There seems to be a barrier at the Upper Devonian level which prevents hydrocarbons from lower levels migrating to higher stratigraphic levels.

The analysed source rocks identified in this article have the potential to generate and expel all the oils with which they are correlated. Of the areas, by far the largest potential is calculated to be in the Pechora-Kolva aulacogen, which fits with the observed yields in the area.

Biodegradation does not occur in the investigated reservoirs.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah