Abstract: Evaluation of Source Rocks, Oils and Reservoir Alteration in Yamal and Gydan Area, Western Siberia

BJOROY, MALVIN, KJELL ARNE BAKKEN, and PETER B. HALL, Geolab Nor AS; ALLA S. ROVENSKAYA, VNIIGeosystem; NIKOLAY NEMCHENKO, State Commission of Russian Federation for Mineral Reserves; FAIZ Z. KHAFIZOV, Tyumen Geological Committee

Introduction

A detailed petroleum geochemical study has been performed over the Yamal and Gydan areas of Western Siberia involving analysis of 57 oils and 48 sediment (core) samples. Oil and gas reservoirs have been found in strata ranging from Jurassic to Cretaceous age. Although most fields in these areas are gas fields, oil samples from them have been analysed since these will give a better understanding of the type of source rocks that have generated the hydrocarbons than will analysis of gas.

Oils

The geochemical data indicate that the analysed oils comprise two main families. Oil family 1 is found in the southwestern part of the studied area, while oil family 2 is found in the northeastern part.

Most of the oils found in reservoirs older than the topmost part of the Upper Jurassic sequence belong to one oil family, and are classified as oil family 2. These reservoirs are also concentrated in the northeast.

Oils found in younger reservoirs belong to oil family 1. The density of these oils tends to be higher than for oil family 2, but the variations are minor with API ranging from 41° to 33°.

The estimated maturity of the oils show a significant variation from 0.7 % to 1.3 % Ro vitrinite reflectance equivalent. This shows that the maturity ranges from early oil window to condensate window.

Reservoir temperatures vary fairly constantly with depth, but in the northeastern part (areas with permafrost) the reservoirs have lower temperatures at comparable depths than in other areas.

The depth to the top of the reservoirs is very variable. Some of the reservoirs are very shallow (<1000 m), others are much deeper (about 3000 m).

Most of the analysed oils are biodegraded and the data from this study indicates that biodegradation can be expected in any fields shallower than 2200 m.

Most of the variations in measured parameters, such as viscosity and density and also chromatographic variation, are attributed to the biodegration level of the samples. Biodegradation gives rise to larger variations in the oils than do differenees in the source rocks. Biodegradation is also believed to have affected the gas/oil ratio, causing an increase in the gas/oil ratio by converting oil and condensate to gas.

Source rock analysis was undertaken on a total of 48 core samples. The source rocks fall into two main groups, which can be correlated to the oil families mentioned above.

Source rock group 1 is the Upper Jurassic Bazhenov Formation and its equivalents. This is by far the best source rock in the area and can be seen to be the dominant source for oil in all areas where it is present. It is also a significant source for gas where the burial depth is great. Source rock group 1 (Bazhenov Formation and equivalents) correlates with oil family 1. Oil family 1 is also found in areas where the Bazhenov Formation and equivalents are known to be present (in the southwestern part of the studied area).

In areas outside the occurrence of the Bazhenov Formation and equivalents, Middle and Lower Jurassic Formations source rocks-group 2 - are the most prominent source rocks. This source rock group is more gas-prone than source rock group 1 and is mainly a source for gas. Source rock group 2 can be correlated with oil family 2. The oils correlated with source rock group 2 are located deeper than those correlated with source rock group 1 (which is stratigraphically higher).

No indication was found in this study that rocks younger than the Bazhenov Formation have source rock potential or are mature enough to have generated significant amounts of hydrocarbons.

Conclusions

Two main oil families have been identified. Oil family 1 is located in the southwestern part of the studied area and oils belonging to this family are mostly found in post-Jurassic reservoirs. Oil family 2 is located in the northeastern part of the studied area and the samples belonging to this family are mostly found in pre-Upper Jurassic age (topmost part of Upper Jurassic).

Two source rock families have been identified. Source rock group 1 consists of shales of the Upper Jurassic Bazhenov Formation and equivalents. This source rock group is found in the southwestern part of the studied area. Source rock group 2 consists of shales of Middle and Lower Jurassic age. This source rock only appears to be important where source rock group 1 is missing, i.e. in the northeastern part of the studied area. This is a mostly gas prone source rock.

Source rock group 1 is correlated to oil family 1. Source rock group 2 is correlated to oil family 2. This means that oils in the southwest are derived from the Bazhenov Formation and equivalents, while oils in the northeast are derived from Middle and Lower Jurassic shales. No other source rocks were identified that could have produced oil or gas.

Biodegradation is very common and oil in reservoirs shallower than 2200 m can be expected to be biodegraded. Biodegradation is believed to have increased the gas/oil ratio compared to the original ratio for hydrocarbons generated from source rock.

Due to the gas prone nature of the source rocks in the northeastern part of the studied area, it is predicted that prospects in this area will contain only minor amounts of oil and will be mostly gas filled.

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