Page 122 - Origin and Prediction of Abnormal Formation Pressures

102 L.A. BURYAKOVSKY, R.D. DJEVANSHIR, G.V. CHILINGAR, H.H. RIEKE III AND J.O. ROBERTSON, JR.
Porosity,

0.001 2 3 4 56 0.01 2 3 4 56 0.1 2 3
o I I I I I I I II I I [ I I II II
IE
,,,,.
:::I:::
i 7
.,,,..
"- 2000 ,2 ~

0 6, 'S
r,,
r
400C

6000
Fig. 4-5. Relationship between porosity (r %) and depth of burial (H, m) for shales. 1 - Devonian
(Weller, 1959); 2 = Mesozoic (Proshlyakov, 1960, and Dobrynin, 1970); 3 = Oligocene to Miocene
(Vassoyevich and Bronovitskiy, 1962); 4-6 -- Middle Pliocene (Durmishyan, 1973a,b) (4 - Apsheron
Archipelago; 5 = South Apsheron Offshore Zone; 6 -- Baku Archipelago and Lower Kura region).
(Modified after Buryakovsky et al., 1995, fig. 3, p. 206.)

4.0-5.5 km is several times higher than in consolidated shales present in other regions
(Fig. 4-5). Such a difference is due to geological age, relative contents of clay and sand,
temperature and other factors. The abnormally high porosity of Apsheron Archipelago
shales is primarily a result of slower rate of compaction as compared to the subsidence
rate, due to the slow pore water removal from the compacting argillaceous rocks during
rapid sedimentation rate. This process was critical in the development of AHFP in the
South Caspian Basin.
Numerous initial formation pressure measurements in reservoir rocks and wireline
logging determination of pore pressure in argillaceous rocks reveal a pattern of AHFP
distribution throughout the section at the northwest flank of the South Caspian Basin
(Table 4-2). The average gradients of initial formation pressures in the reservoir rocks,
r/res, and of pore pressures in shales, rich, at the investigated depths, are (in MPa/m):
0.0106 and 0.0120 for the Apsheron Archipelago; 0.0119 and 0.0145 for the South
Apsheron Offshore Zone; and 0.0138 and 0.0182 for the Baku Archipelago and Lower
Kura region. A substantial difference between the initial formation pressures in reservoir
rocks and pore pressures in shales (by a factor of over 1.5) exists in the Baku
Archipelago, where the average thickness of shales, hsh, is particularly higher than in the
other regions of Azerbaijan. Generally, AHFP rises with the relative content of shales,
Xsh, throughout the section (Table 4-2) and within the reservoir (Fig. 4-6). The highest
shale pore pressures are associated with shale sequences in the Baku Archipelago