لخّصلي

logging responses corresponding to each pore structure type and divided the pore struc-
ture types qualitatively. Zhang Xinwen et al. [15] combined nuclear magnetic resonance
(NMR) logging data with high-pressure mercury injection data to obtain continuous pore
structure parameters.
With the application of the studies mentioned above, some scholars have made certain
achievements in the characterization and evaluation of reservoir pore structures. However,
there are still some deficiencies in existing technical methods due to a wide range of pore
sizes, strong heterogeneity, and complex pore structure in carbonate reservoirs. Firstly,
the characterization and classification of reservoir pore structures are not detailed enough.
They are mostly based on factors such as pore-throat size or pore-throat distribution char-
acteristics, without comprehensively considering the subtle differences in pore structure
reflected by multiple factors. Secondly, the classification and evaluation of reservoir pore
structure are usually completed through core experiments whereas the core data is usually
limited. Logging data is therefore required to predict the type of reservoir pore structure.
Most of the existing classification and evaluation work is completed by qualitative methods
or linear regression models with rather limited accuracy. Finally, although nuclear magnetic
resonance (NMR) logging data is reliable for constructing a pseudo capillary pressure curve
for calculating pore structure parameters, it is not a widely-applied conventional logging
method. Many old wells and development wells do not have such logging projects. The
cost of acquiring nuclear magnetic resonance (NMR) logging data is also too high for
large-scale applications. Taking the North Truva Oilfield in the Pre-Caspian Basin as an
example and referring to the previous technical ideas in hydrocarbon development [16–19],
in this paper, (1) the pores and throats are studied by cores, cast thin sections and scanning
electron microscopy to define the types and criteria of pore configuration. (2) Different
petrophysical facies were divided based on mercury injection parameters and shapes of
capillary pressure curve shapes. (3) They were then identified and predicted with high
precision through machine learning methods for the wells in the oilfield. (4) A complete
methodology was established for characterizing and evaluating the pore structure of com-
plex carbonate reservoirs, which is of great significance to the efficient development of
such oilfields.
2. Geological Overview of the Study Area
The Pre-Caspian Basin, also known as the North Caspian Basin, is located in the
northwest, north and northeast of the Caspian Sea, spanning the territory of Kazakhstan
and Russia. The Caspian Basin extends in the EW direction, with a length of 1000 km
in the EW direction and a width of up to 650 km in the NS direction. The outline of the
basin is approximately oval. It covers an area of 50 × 104 km2
, of which about 85% is
located in western Kazakhstan and 15% is located in southern Russia. Geotectonically, the
Pre-Caspian Basin is located in the southeastern part of the Eastern European platform. It
extends to the Ural Hercynian fold belt in the east. It has been a superimposed basin since
the Late Proterozoic. The interior of the basin can be further divided into four secondary
tectonic units, including the fault terrace belt (north and northwest), the central depression
belt, the southeast uplift belt and the southern uplift belt, together with a large number of
secondary structural units (Figure 1). The Pre-Caspian Basin is a world-famous super-large
hydrocarbon-bearing basin [20–22]. So far, a series of large and super-large carbonate
oil and gas fields have been discovered under the Permian salt layer, such as Astrakhan,
Zanarol, Tengiz, Kenkiyak and Kashagan.
The North Truva Oilfield is located on the eastern margin of the Pre-Caspian Basin and
is a large weakly volatile complex carbonate reservoir with a condensate gas cap [23]. The
oilfield is located in the central block of the eastern slope of the Yanbek-Zarkames paleo-
uplift in the eastern margin of the Pre-Caspian Basin, which is a NE-SW trending faulted
anticline [24]; there are 10 faults developed in North Truva Oilfield, mainly in the NE
strike. The structure is cut into 10 fault blocks by faults (Figure 2). In the oilfield, the strata
revealed by drilling are Quaternary to Devonian. Salt dome structures are