Compaction of Clays as a Factor in the Formation of Hydrodynamic Regime and Hydrocarbon Migration and Accumulation

Abstract

Based on extensive experience, the writers present relationships (1) between the porosity and depth of burial, and (2) between the permeability and depth of burial in undercompacted thick argillaceous sequences. They also explain the formation of abnormally high formation pressures (AHFP) and mode of oil/gas migration and accumulation. Special emphasis is placed on petroliferous deposits of Azerbaijan.

Keywords:

• Azerbaijan
• compaction of clays
• hydrocarbon migration
• hydrodynamics

Notes

¹Infiltration basins are subsidence areas within tectonically stable regions, mostly platforms. They represent relatively shallow depressions with the artesian hydrodynamic regime. Typical examples are some platform depressions of Russia, such as the Moscow, N. Dvina, Vyatka-Kama syneclises, Angara-Lena Depression, Kyzylkum Depression of the Central Asia and some others. They usually have relatively thin sediment cover (2 to 3 km) which reflects sluggish tectonic regime and the proclivity to epeirogenic motions.

The sediment cover of these “tectonic bowls” is usually uncovered by erosion. Vadose waters from the surface penetrate the reservoir rocks and migrate to the discharge areas where the most and the role of clay sequence compaction in initiating the fluidal flows and formation and distribution of oil and gas accumulations.

It is important to define a few terms prior to initiating discussion: (1) Geodynamic (i.e., gravity, compression, overburden) clay compaction, which is a function of overburden continuously increasing in time. (2) Geostatic (i.e., decompression, unloading) process, which is associated with the relaxation of cumulative energy in the fluids compressed in pores (energy accumulated over the previous geologic evolution). (3) Geotectonic compaction which is of tectonic (stress) origin.

The first mechanism is typical for the modern geologic environment over the areas of extended compensated deposition, subsidence, and permanent immersion. These are mostly shelf zones and offshore areas in the internal sea basins. The second mechanism is characteristic of the dry land areas where the current continental deposition does not compensate for the clay sequence subsidence, so there is, to a substantial extent, preservation of inherited abnormally-high porosity (water-saturation). The last one is a feature of belts and zones of powerful neo- and paleotectonic events.

The geodynamic and geotectonic mechanisms eventually result in the formation of shales, lamination, tabulation, and fracturing. The geostatic compaction does not significantly change the clay pore space morphology, and clays mostly preserve their structure.

Regardless of mechanism involved, there is a clay drainage and emigration of the compressed pore waters from their pore space. The following factors control the nature and speed of this process in a specific geologic environment:

1. Rate and depth of clay subsidence.

2. Thickness of clay sequences.

3. The sign of regional tectonic movements and intensity of the tectonic stresses.

4. The presence and duration of interformational depositional interruptions.

5. Extent of the filtration, fracture, diffusion, and film permeability of clay varieties.

6. Spatial variability of the clay mineralogical composition and lithology.

7. Contents of sand-silt, carbonate, and siliceous material.

8. Degree of diagenetic and catagenetic alterations of clays.

9. Clay mineral composition and efficiency of dehydration mechanisms.

10. Ratio of tight to reservoir intervals, which serve as drainage (discharge) avenues, in the compacted sequence.

11. Intercommunication of permeable intervals with the near-surface intervals and the surface (along faults, for example).

12. Geologic age of compacted intervals and duration of their residence at a certain depth and temperature.

13. The directions (vectors) of stress.

∗Calculated value.

∗∗KZ, Cenozoic; N₂, Upper Neogene; N₁, Lower Neogene; Pg₃, Upper Paleogene; K₂, Upper Cretaceous; K₁, Lower Cretaceous; PZ, Paleozoic.

∗For Darcy's equation calculations, it was assumed in the first two cases that the “neutral” surface (boundary between water squeeze-up and down) is positioned in the middle of the clay sequence. Pressure gradient over a distance of 1,000 m was assumed to be 10 MPa.

∗For Darcy's equation calculations, it was assumed in the first two cases that the “neutral” surface (boundary between water squeeze-up and down) is positioned in the middle of the clay sequence. Pressure gradient over a distance of 1,000 m was assumed to be 10 MPa.