Timing Of Petroleum Migration Research Articles

Relationships between seepage, tectonics and subsurface petroleum reserves

The relationship between visible petroleum seepage and commercial subsurface reserves has been investigated by a study of the distribution of documented seeps in south-east Asia. This is accompanied and integrated with a broader global study. The intensity of flowing seepage is controlled largely by present day tectonics and by the timing of petroleum migration. When such influences are reduced by isolating sets of tectonically similar basins, correlations are found between numbers of seeps and basin reserves. At a sub-basinal scale, seeps are observed to concentrate over tectonic features such as active diapirs, active faults and uplifted basin margins. It is relatively rare for seeps to directly overlie major fields, although many seeps may be linked to downdip accumulations by long tortuous paths. The best seep-accumulation relationships are observed in active compressional and diapiric settings, particularly those involving shallow reservoirs. Seeps are rare where traps are buried beneath thick undisturbed overburdens. The prime value of visible seeps in most frontier basins lies at the regional level, in the clues they give to the nature, extent and quantitative hydrocarbon potential of a basin's source system.

K/Ar Dating of Illitic Clays in Jurassic Nugget Sandstone and Timing of Petroleum Migration in Wyoming Overthrust Belt: ABSTRACT

Authigenic illite is a prominent pore-fill in the Nugget Sandstone, the main reservoir rock of most fields in the southwest Wyoming Overthrust belt. Illite, a good K/Ar clock, was dated from several well samples, all from the Absaroka thrust sheet. This includes a producing well in the Clear Creek field where seven samples traverse the gas, oil, and water zones. The ages of the Clear Creek suite are virtually concordant at 110 ± 2 m.y. Assuming hydrocarbon emplacement would have arrested authigenesis in the oil and gas zones, the similarity of ages from the hydrocarbon zones with the water zone indicates hydrocarbon emplacement was post 110 m.y. ago (middle Cretaceous). Ages obtained from the other Absaroka sheet Nugget samples fall in the narrow range of 102 to 120 .y.b.p. This indicates illite authigenesis was a relatively short-lived event for the Nugget in the Absaroka sheet. The Wyoming-Idaho-Utah overthrust belt involves several thrust sheets each of which was emplaced over its foreland sequentially from west to east over a time spanning tens of millions of years. We attribute the mid-Cretaceous illite growth in the Absaroka sheet to burial conditions established when that part of the Nugget was thrust upon by the Crawford sheet. The burial was accomplished both tectonically and by synorogenically derived sediment. If true, our illite dates imply a somewhat older age for the Crawford sheet than previously interpreted. We attribute the post-illite hydrocarbon emplacement in the Absaroka Nugget as a result of thrusting of the Absaroka sheet on top of its foreland containing middle Cretaceous petroleum source-bed shales. These beds were thermally matured wh n buried by the emplacement of the Absaroka sheet and its derived sediment in the Late Cretaceous. End_of_Article - Last_Page 414------------

K/Ar Dating of Illitic Clay in Sandstone Reservoirs and Timing of Petroleum Migration: ABSTRACT

Illite and illite/smectite, which occur as authigenic pore-fill in some sandstones, are excellent argon-retentive clocks. Dating their mean time of growth in conjunction with other basin information may add to predictive understanding of illitic cement distribution. Paragenesis and distribution of authigenic clay in the sandstone body with respect to hydrocarbon occurrence may allow the measured age to be related to time of hydrocarbon emplacement. For certain petroleum-source shales, the mean age of illite/smectite burial metamorphism, and thus petroleum maturation, can be measured directly and then compared to the age of clay authigenesis in reservoir sandstones to see if the two phenomena are related. Three sandstones have been examined: the Cretaceous Muddy on the eastern flank of the Powder River basin; the Jurassic Nugget of the Wyoming Overthrust belt; and the Permian Rotliegendes of northern Europe. For the regressive poorly sorted Muddy, logistic problems arise because of the low abundance of authigenic illite/smectite compared to kaolinite and the common occurrence of old detrital illite. The age of illite/smectite authigenesis thus is asymptotically derived from analyses of a series of progressively finer (and authigenically purer) clay size fractions down to < 0.05µ. Over a broad region, this derived age is about 40 m.y., excluding Bell Creek which is anomalous. This age may associate illite/smectite growth in the reservoir with the early Tertiary deep burial of th basin interior. Because similar ages occur in both oil and water zones, illite/smectite growth may either have preceded or accompanied oil emplacement. Logistic problems are minimized for the abundant, high-K2O, discrete illite cements of the well-sorted aeolian Nugget and Rotliegendes. For both of these, precise sets of ages have been obtained for individual fields in both water and hydrocarbon zones, implying cementation may have been a relatively short-lived event. The Nugget samples, all from the Absaroka sheet, give late Mesozoic ages. In the Rotliegendes, complex block faulting has led to unique post-late Jurassic burial and gas generation histories for adjacent fault blocks. However, thus far the three areas of the Rotliegendes examined indicate illite growth largely preceded this history. End_of_Article - Last_Page 1442------------