TY - JOUR
T1 - Source rock quality variations of upper Jurassic - lowermost Cretaceous marine shales and their relationship to oils in the Valdemar Field, Danish North Sea
AU - Ponsaing, L.
AU - Petersen, H.I.
AU - Bojesen-Koefoed, J.A.
AU - Nytoft, H.P.
AU - Schovsbo, N.H.
AU - Stemmerik, L.
N1 - Publisher Copyright:
© 2020 The Authors. Journal of Petroleum Geology © 2020 Scientific Press Ltd
PY - 2020/1/1
Y1 - 2020/1/1
N2 - The main source rocks for the hydrocarbons at the Valdemar field (Danish North Sea) are the Upper Jurassic – lowermost Cretaceous organic-rich marine shales of the Farsund Formation. However, geochemical analyses of retained petroleum in reservoir cores show variations in oil type and maturity which indicate a complex charging history. This paper reviews the organofacies and source rock quality variations in 55 samples of the Farsund Formation from the North Jens-1 well (Valdemar field) within a sequence stratigraphic framework in order to discuss the source of the hydrocarbons. Petrographic and geochemical data, including biomarker analyses, were integrated in order to characterize the kerogen composition, original source rock potential and depositional environment of the Farsund Formation. The thermal maturity, source rock quality and kerogen quality all vary at the sequence level, and in general change upwards from early mature, primarily gas-prone Type II kerogen in the Kimmeridgian Kimm-2 and Kimm-3 sequences to immature, highly oil-prone sapropelic Type II kerogen in the Volg-4 and Ryaz-1 sequences (Volgian, Ryazanian). The kerogen has a maceral composition dominated by fluorescing amorphous organic matter (AOM) and liptodetrinite, with variable but generally minor amounts of terrigenous organic matter. The stratigraphic distribution of organic matter is similar to that in regional observations from the Danish Central Graben but minor differences occur, especially in the amount of fluorescing AOM in the Kimmeridgian sequences. The decrease in terrigenous input (vitrinite) upwards through the marine shale succession likely reflects a marine transgression of the Danish Central Graben area during Late Jurassic time. The source potential of the Upper Jurassic – lowermost Cretaceous shales in the North Jens-1 well is generally lower than that observed regionally, including an absence of relatively organic-rich, oil-prone intervals in the older part of the succession which have been demonstrated to occur elsewhere in the Danish Central Graben. However, in agreement with the regional trend, back-calculated source rock data and calculated Ultimate Expulsion Potentials show that the uppermost Volgian (Volg-4) and Ryazanian (Ryaz-1) sequences are the most oil-prone intervals. The Ryaz-1 sequence represents a condensed section formed during a period characterised by low sedimentation rates and high preservation of algal organic matter. Biomarker compositions from source rock extracts from the North Jens-1 well cannot be directly correlated to Valdemar reservoir oils, suggesting that the mature organofacies which charged the oils are not represented in the samples from North Jens-1.
AB - The main source rocks for the hydrocarbons at the Valdemar field (Danish North Sea) are the Upper Jurassic – lowermost Cretaceous organic-rich marine shales of the Farsund Formation. However, geochemical analyses of retained petroleum in reservoir cores show variations in oil type and maturity which indicate a complex charging history. This paper reviews the organofacies and source rock quality variations in 55 samples of the Farsund Formation from the North Jens-1 well (Valdemar field) within a sequence stratigraphic framework in order to discuss the source of the hydrocarbons. Petrographic and geochemical data, including biomarker analyses, were integrated in order to characterize the kerogen composition, original source rock potential and depositional environment of the Farsund Formation. The thermal maturity, source rock quality and kerogen quality all vary at the sequence level, and in general change upwards from early mature, primarily gas-prone Type II kerogen in the Kimmeridgian Kimm-2 and Kimm-3 sequences to immature, highly oil-prone sapropelic Type II kerogen in the Volg-4 and Ryaz-1 sequences (Volgian, Ryazanian). The kerogen has a maceral composition dominated by fluorescing amorphous organic matter (AOM) and liptodetrinite, with variable but generally minor amounts of terrigenous organic matter. The stratigraphic distribution of organic matter is similar to that in regional observations from the Danish Central Graben but minor differences occur, especially in the amount of fluorescing AOM in the Kimmeridgian sequences. The decrease in terrigenous input (vitrinite) upwards through the marine shale succession likely reflects a marine transgression of the Danish Central Graben area during Late Jurassic time. The source potential of the Upper Jurassic – lowermost Cretaceous shales in the North Jens-1 well is generally lower than that observed regionally, including an absence of relatively organic-rich, oil-prone intervals in the older part of the succession which have been demonstrated to occur elsewhere in the Danish Central Graben. However, in agreement with the regional trend, back-calculated source rock data and calculated Ultimate Expulsion Potentials show that the uppermost Volgian (Volg-4) and Ryazanian (Ryaz-1) sequences are the most oil-prone intervals. The Ryaz-1 sequence represents a condensed section formed during a period characterised by low sedimentation rates and high preservation of algal organic matter. Biomarker compositions from source rock extracts from the North Jens-1 well cannot be directly correlated to Valdemar reservoir oils, suggesting that the mature organofacies which charged the oils are not represented in the samples from North Jens-1.
KW - biomarker
KW - Danish Central Graben
KW - kerogen
KW - marine shales
KW - North Sea
KW - organofacies
KW - Source rock
KW - Upper Jurassic – lowermost Cretaceous
KW - Valdemar field
UR - http://www.scopus.com/inward/record.url?scp=85076891335&partnerID=8YFLogxK
U2 - 10.1111/jpg.12749
DO - 10.1111/jpg.12749
M3 - Article
AN - SCOPUS:85076891335
SN - 0141-6421
VL - 43
SP - 49
EP - 74
JO - Journal of Petroleum Geology
JF - Journal of Petroleum Geology
IS - 1
ER -