Hydrocarbon migration behaviour in Late Jurassic reservoirs of Kaymisovsk arch

Khromovskikh A.Y.

Joint Stock Company TomskNIPIneft, Tomsk, Russia

ahrom@sibmail.com

A thorough study of geologic structure of an area disputes formerly popular ideas about deposit formation in Kaymisovsk arch through lateral hydrocarbon migration from submerged zones of east-neighboring Nurol basin. Thus, there is a little chance for hydrocarbon migration from the area to deformational traps of Kaymisovsk arch as well as the possibility of oil deposit formation. Submerged zones and wing areas of positive structures of the basin are mostly lacking a durable reservoir that contributes to the accumulation of hydrocarbons coming from overlying oil source deposits of Bazhenov formation rocks and assures the secondary hydrocarbon migration to the traps. If late Jurassic reservoir is observed in these areas it generally has a conductivity rising to the first units of micrometres²·10^-3 and its horizontal facial variation is very high. The absence of the late Jurassic reservoirs in submerged zones which assure accumulation of hydrocarbons migrating from Bazhenov formation disputes an earlier conclusion about a high degree of hydrocarbon generation in the Bazhenov formation deposits [1].

Analysis of Bazhenov formation capacity for generation made in Kaymisovsk arch deposits showed that actually identified oil-in-place resources may be generated by the deposits of Bazhenov formation within known oil-bearing contours.

When the primary migration is taking place, hydrocarbon molecules remain in homogeneous mixture with the molecules of interstitial water without phase separation. As a consequence, period they migrate easily from oil-source mass completed with forced-out interstitial water in the faluation period due to generationally raised pore pressure and lithostatic pressure of pore fluids.

The flow is directed by the zone of pressure relief that represents a porous or a joint-liked medium of underlying late Jurassic reservoirs of horizon U₁. In addition to this, generation, emigration and migration processing of hydrocarbons are accompanied with their accumulation [2].

As A. Levorcen affirms [3], every deposit is a singular phenomenon and its formation is to consider as a final result of interaction of 20-25 variable quantities. The most significant impact on deposit formation in the area is exerted by such factors as lithologic and facial properties of reservoirs and their filtration and capacitive behaviour, joint-liked structure of reservoirs, particularities of the structure of the pore space, the show of superimposed catagenesis within oil deposit formation, regional hydrodynamic pressure of deposit water.

The analysis of the impact of some factors allowed noticing that regional hydrodynamic pressure of deposit water plays a questionable role in oil deposit formation. The vertical top-down migration of homogenous mixture of hydrocarbon fluids and pore water in reservoir is also influenced by alongside hydrodynamic pressure of deposit water. The interaction of vertical and horizontal pressure explains the degree of hydrocarbon migration to the pore space of reservoir. Hydrocarbon migration in wing zones of traps, affected by increased hydrodynamic pressure (east, south), is much less because the pressure makes an obstruction to the vertical migration of Bazhenov formation deposits. Quite a different thing is to be observed on the opposite wings (west, north). Hydrodynamic pressure in these zones is much less, so that contributes to the extent of homogenous mixture of hydrocarbon fluids and pore water to the submerged zones of wings. Thus, the migration of the oil from Bazhenov formation in the north-west and west areas of traps was structurally lower than in the east and south-east areas that led to the formation of inclined water-oil contact (WOC).

Filtration and capacitive behaviour of reservoirs that are situated at the bottom of oil source deposits of Bazhenov formation takes a specified part in formation of late Jurassic deposits and inclined WOC. It becomes obvious if we take the example of one of the north-west deposits in Kaymisovsk arch. The north and north-east areas of the deposit show a structurally high position of WOC at absolute depth marks of -2412-2421 m that goes contrary to the regular low of WOC due to the regional hydrodynamic pressure. This may be justified by the reservoirs’ behaviour from coal-overlaying unit of horizon U₁ where hydrocarbon migration is taking place_. If most of deposit sandstones have the total net reservoir thickness of 8-12 meters, the north and north-east areas demonstrate the decrease of net reservoir thickness to 2-3- meters and the deterioration of their filtration and capacitive behaviour due to the shale index. It is an increased capillary pressure in shaly reservoirs with a low conductivity that constitutes an additional resistance leading to the decrease of hydrocarbon migration speed on the area of the upper Bazhenov formation, despite the favourable conditions of hydrodynamic pressure of deposit water.

The similar thing is to be found on the main reservoir (deposit J₁³) of the south deposit on Kaymisovsk arch. Along with a regular low of WOC from the south to the north ranging between -2602 and -2620 m, the north area demonstrates the range of absolute depth marks within -2566-2581 m. The main reason for this is a plenty of reservoirs with low filtration and capacitive behaviour, which have conductivity twice as small as main reservoirs of deposit that is specified by facial formation of deposit J₁³.

The same phenomenon of various position of WOC is observed on a neighboring deposit, situated to the south of an examining deposit. The reservoirs of deposit J₁³ were formed in similar conditions. Apart from this, water-saturated reservoirs are to be found within oil deposit, in the area of low-permeability sandstones, at structurally high levels. It is caused by a weaken hydrocarbon migration from Bazhenov formation or by its absolute absence. An additional resistance to the migration that is stipulated by an increased capillary pressure in shaly reservoirs may be a substantial reason for that.

Conclusion

Kaymisovsk arch deposits were formed owing to hydrocarbon migration generated by the deposits of Bazhenov formation and those within known oil-bearing contours.

The migration of hydrocarbons from Bazhenov formation in the north-west and west areas of arch deposits was structurally lower under the effect of the regional hydrodynamic pressure of deposit water than the migration in the south-east and east areas.

Filtration and capacitive behaviour of reservoir that is heterogenous by its surface and cut explains a different speed of reservoir areas filling by hydrocarbons migrated from the overlying deposits of Bazhenov formation.

References:

1. Khromovskikh A.Y., Voloshchuk G.M. The particularities of late Jurassic oil deposit formation in the south-east of Western Siberian Plate. // Tomsk polytechnic university journal. - 2011. Vol.318. №1. – P.103-106.

2. Zapivalov N.P., Khromovskikh A.Y. Migration behaviour during late Jurassic oil deposit formation in Kaymisovsk Arch, West Siberia // DEW: Drilling and Exploration World. – India: 2011. - Vol. 20., № 6, April. – P. 35–40.

3. Levorsen А. Petroleum geology. – М.: Gostoptechizdat , 1958. – 487 p.

Scaling relationships of faults and fractures crosscutting Cretaceous platform carbonates in the Murge Plateau (southern Italy): implications for the characterisation and modelling of deformed carbonate reservoirs

Korneva I.¹, Tondi E.¹, Agosta F.², Cuia R.D.³, Bitonte R.³, Giorgioni M.⁴

¹Geology Division, School of Science and Technology, University of Camerino, Italy; ²Department of Earth Sciences,University of Basilicata, Italy; ³G.E.Plan Consulting, Ferrara, Italy; ⁴Shell E&P, Rome, Italy