Predicted bulk composition of petroleum generated by Lower Cretaceous Wealden black shales, Lower Saxony Basin, Germany

Volker Ziegs,Benjamin Bruns,Brian Horsfield,Nicolaj Mahlstedt

doi:10.1007/s00531-014-1081-y

Abstract

The Berriasian Wealden Shale provides the favourable situation of possessing immature to overmature source rock intervals due to differential subsidence within the Lower Saxony Basin. Hydrocarbon generation kinetics and petroleum physical properties have been investigated on four immature Wealden Shale samples situated in different depth intervals and following the PhaseKinetics approach of di Primio and Horsfield (AAPG Bull 90(7):1031–1058, 2006). Kinetic parameters and phase prediction were applied to a thermally calibrated 1D model of the geodynamic evolution at the location of an overmature well. The immature source rocks of all depth intervals comprise kerogen type I being derived from the lacustrine algae Botryococcus braunii. Bulk kinetics of the lower three depth intervals (sample 2–4) can be described by one single activation energy E a, typical for homogeneous, lacustrine organic matter (OM), whereas sample 1 from the uppermost interval shows a slightly broader E a distribution which hints to a more heterogeneous, less stable OM, but still of lacustrine origin. Predicted physical properties of the generated petroleum fluids are characteristic of variably waxy, black oil possessing GOR’s below 100 Sm3/Sm3 and saturations pressures below 150 bar. Petroleum fluids from the more heterogeneous OM-containing sample 1 can always be described by slightly higher values. Based on the occurrence of paraffinic, free hydrocarbons in the uppermost horizon of the overmature well and gas/condensate in the lower 3 depth intervals, two scenarios have been discussed. From the first and least realistic scenario assuming no expulsion from the source rock, it can be deduced that phase separation in the course of uplift can only have occurred in the uppermost interval containing the slightly less stable OM but not in the lower intervals being composed of a more stable OM. Therefore and taking secondary cracking into account, all depth intervals should contain gas/condensate. The free hydrocarbons in the upper horizon are interpreted as impregnation from migrated hydrocarbons. The second scenario assumes nearly complete expulsion due to fracturing by the so-called generation overpressure (Mann et al. in Petroleum and basin evolution. Springer, Berlin, 1997). The expelled petroleum might migrate into lower pressurised source rock horizons and reach bubble-point pressures leading to the exsolution of gas and “precipitation” of very high molecular weight bitumen unable to migrate. Subsequent burial of the latter in the course of the basin evolution would lead to secondary cracking and remaining pyrobitumen explaining the high amounts of pyrobitumen in the overmature well Ex-B and relatively enhanced TOC contents at such high maturity levels.

Highlights

The Lower Saxony Basin (LSB) is the most important oil province of Germany as well as the oldest oil-producing basin in the world, with first production starting as early as 1859
Sample 1 shows a marine signature, at least in the classical sense (Tegelaar and Noble 1994; Pepper and Dodd 1995), by displaying a relatively broad Ea distribution with values ranging between 44 and 64 kcal/mol and a main energy at 55 kcal/mol accounting for 45 % of the total kerogen to petroleum conversion reaction, whereas Wealden Shale samples 2–4 from the lower depth intervals display an Ea distribution which is dominated by one single Ea, between 58 and 60 kcal/mol, accounting for at least 96 % of the total kerogen to petroleum conversion reaction
Domination of a single activation energy is commonly thought to be characteristic for lacustrine type I source rocks (Pepper and Corvi 1995) and reflects a homogeneous mainly polymethylene kerogen structure which is most likely inherited by selectively preserved aliphatic algaenan, the resistant biopolymer from within the outer cell walls of microalgae, such as B. braunii, and comprises only a limited range of chemically stable bonds

Summary

Introduction

The Lower Saxony Basin (LSB) is the most important oil province of Germany as well as the oldest oil-producing basin in the world, with first production starting as early as 1859. The hydrocarbon fields are more or less aligned in a NW–SE direction in the LSB, an N–S trending region in the Gifhorn Trough and a NE–SW trend in the West and East Holstein Troughs (including the Mittelplate oil field). The crude oils are derived from two sources, namely the Wealden and the Posidonia Shale (e.g. Gifhorn Trough, Schwarzkopf and Leythaeuser 1988) with co-sourced petroleums being commonplace. All oil fields occur in the source rock maturity zone 0.5–0.9 % R0 (Kockel et al 1994). Petroleum sources are readily distinguished by stable isotopic composition, the isotopically lighter signature being attributed to Posidonia Shale and the heavier to Wealden (Kockel et al 1994). The contribution of the two sources may be reflected in the composition of the asphaltenes of migrated bitumens in the Hils Syncline, as seen in the Holzener Asphaltkalk (Horsfield et al 1991)

Methods

Results

Discussion

Conclusion