Prediction of abnormally high formation pressure zones using 3D seismic inversion data

Abstract

Background. The article provides information about the location of the Hovsan field under study, the history of its investigation by various geological and geophysical methods, and the main results obtained. It is noted that the study of this area began as early as the 19th century from the adjacent Surakhany-Karachukhur area, where the first exploratory well was drilled (Karachukhur area). From the 1930s to the 1940s, various geophysical surveys were also carried out within the study area, and as a result of electrical prospecting, the first understanding of the tectonics of the Bina-Hovsan syncline was obtained. Gas survey data later made it possible to identify anomalous zones within the Hovsan field. Seismic exploration was conducted from the 1930s to the mid-1950s using the reflection-seismic method (RM), and in 1977–1978 using the common depth point (CDP) method. Individual seismic profiles were acquired in 1993 and 1999. Detailed CDP surveys were carried out in 1996 and in 2003-2004. In 1948, oil was discovered in well No. 1308 in the Kalin Suite of the Productive Series, after which the Hovsan oil field was put into commercial development. The field is currently exploited only onshore, since its offshore boundaries remain undefined due to insufficient study.

Methods. In 2011–2012, 3D CDP seismic surveys were carried out in the Zykh-Hovsan area. These surveys were conducted under varying near-surface seismic-geological conditions, covering a total area of 70 km². Information is provided on the field operations: four types of seismic energy sources and three types of recording receivers were used in this area. During the field setup, source matching was not properly implemented, which created difficulties during data processing. In addition, receiver calibration was only partially completed, leading to further complications. Special attention is given to the implementation of seismic inversion. Vertical seismic profiling (VSP) surveys were successfully conducted in two wells of the Hovsan field wells 1856 and 1867. Brief information is presented regarding the borehole surveys and the processing of the acquired data.

Results. Pseudo-acoustic inversion (PAC inversion) was obtained, and a comparison of vertical slices of the absolute and relative acoustic impedance cubes along one of the 3D survey lines of the Hovsan field showed that, in both sections, a strong decrease in acoustic impedance values is observed at the level of the underlying PS deposits. This decrease is mainly associated with a sharp reduction in seismic wave velocities and densities from 110 to 2598 (m/s)·(g/cm³) in clay-rich, fluid-saturated formations, which are typical for zones of abnormally high reservoir pressure (AHRP). The presence of an AHRP zone is also confirmed by the VSP results obtained from the two wells in the field. Surface and borehole seismic studies made it possible to identify an AHRP zone in the Hovsan field section. Stratigraphically, the AHRP interval encompasses the underlying PS deposits, which are represented lithofacially by clay-rich sediments with interbedded sands. These sand interlayers did not appear on conventional seismic sections. However, seismic inversion of the 3D seismic data made it possible to distinguish these sandy intervals.

Conclusions. A very important conclusion is that the AHRP zone is clearly identified on vertical sections of both the absolute acoustic impedance and the relative impedance. Thus, it can be stated that seismic inversion of 3D seismic data can be used to predict the AHRP zone. A well drilled into the Miocene deposits in 2015 confirmed our assumptions and produced a flow of approximately 100 tons of oil per day, further confirming the hydrocarbon potential of the underlying PS deposits within the study area.