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Current Status and Research Methods of Cracked Basement Rock


CHAPTER 2

MODELING METHOD OF FRACTURED POROUSITY IN BLACK LION BASEMENT ROCK


2.1. Overview of fractured basement rock.

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Fractured granitoid basement is a special oil-bearing object in the Cuu Long basin in general and of the Hai Su Den structure in particular. The granitoid itself has no primary porosity and no permeability. The geological processes during and after the formation of rocks form cracks, cavities, driftwood-washed micro-grooves and especially open cracks that create secondary porosity and permeability forming the "fractured granite basement reservoir" with an average porosity of 1-3%, permeability up to thousands of mD in well-fractured zones.

2.1.1. Current status and research methods of fractured basement rock

Current status of research on fractured basement rock

Petroleum in fractured basement rocks was discovered in the early years of the 20th century. To date, petroleum has been discovered in more than 30 countries in the world on most continents and in most types of basement rocks, from young Mesozoic granite rocks (in the Cuu Long basin or other areas in Southeast Asia) to pre-Cambrian metamorphic sediments (some fields in the Middle East such as Azura, Libi) or in the oldest Proterozoic rocks (in the East Siberian region). Some typical examples of petroleum found in basement rocks include: sedimentary basins in Argentina (Cuyo and Neuquen), sedimentary basins in Yemen (DNO lot 43, Nexen lot 14, Total lot 10, ONV lot S2), sedimentary basins in Vietnam (Cuu Long)...

It can be seen that oil and gas are found in basement rocks, in rocks from low-grade metamorphic sediments to high-grade metamorphic sediments or in magmatic rocks. The flow properties in different basement rock groups are also different, in which the largest flow potential is the fractured granitoid basement rock. Some typical deposits of granitoid basement rocks in the world can be mentioned as: Mesozoic granitoid basement rocks of Kora mine in New Zealand.


In Vietnam, large deposits in fractured garnitoid basement include pre-Tertiary granitoid basement at Bach Ho, Rong, Su Tu Den, Su Tu Trang, Su Tu Vang, Hai Su Den, Ca Ngu Vang, Phuong Dong, Rang Dong mines... The largest oil reservoir in basement belongs to Cuu Long basin, with an estimated potential of 6,400 million barrels. Up to now, 25 structures out of a total of 42 structures have been explored in the basement, of which 16 structures have been put into development.

Figure 2.1. Distribution diagram of reserves in the basement in the Cuu Long basin

Methods of studying fractured basement rocks

Vietnam is the country with the world's leading oil production from fractured basement. Oil and gas from fractured basement in Vietnam is mainly exploited from the Cuu Long basin. In this area, the Bach Ho field is a typical example with recoverable reserves of up to 1.4 billion barrels of oil. In addition, there are also the Rong, Rang Dong, Ruby and Su Tu Den fields with reserves from 100 to 400 million barrels of oil [26].

However, research on basement rocks in Vietnam is only at the level of studying the formation of cracks, predicting the existence of fracture zones, permeability characteristics and other factors.


Causes affecting the permeability of fractured basement rocks. Oil and gas exploration and exploitation in fractured basement rocks still face many difficulties in explaining and predicting the distribution and characteristics of fractured zones.

Although oil and gas in basement rocks are very potential, searching, analyzing, and evaluating the recoverable reserves of these fields is very difficult due to the complex nature of basement rocks. There are some cases where the initial production of the field is very high, but it decreases very quickly. The reason is that the porosity system and the circulation properties of oil have not been analyzed, evaluated, and understood. Therefore, building a porosity model for fractured basement rocks is very necessary in evaluating the potential and reserves of these types of fields.

Currently, there are three main methods for constructing a porosity model in basement rock: Halo method, DFN (Discrete Fracture Network) method, and ANN (Artificial Neural Network) method being applied. Each method has different advantages and disadvantages. In addition, the choice of method for model construction is also controlled by the quality and quantity of input data.

Halo method: This is the first method used in Vietnam and is still widely used today, in areas with average to poor seismic data quality. This method mainly relies on fault interpretation, so it depends on the subjectivity of the interpreter. The disadvantage of this method is that it assumes that the fault system is homogeneous along the fault surface. In Vietnam, this method is widely used in the assessment of increased reserves in fractured basement rocks, for example in the Black Lion SW mine [29]. In order to overcome the weaknesses of the Halo method, the improved Halo method is applied by using additional seismic attributes and classifying faults to be able to assess the heterogeneity of fractured porosity and their connectivity along the fault surface.


Figure 2.2. Fine-grained veins (gouges) appearing on the fault surface can act as wedges, preventing the movement of fluids to the upper reservoirs.

DFN (Discrete Fracture Network) method: used when there are many well documents and geological information of the area. The model built by this method is based on statistics of fracture parameters such as: direction, length, opening, shape. This method builds a connection model of faults and fracture systems, from which we get the characteristics of the flow in the reservoir rock. This method was introduced in the 1970s and has been widely used in model building with many improvements throughout the process since then. The DFN method is widely used in research on fractured rocks in general. For example, building a model in fractured basement rocks in Olkiluoto, Finland or in Soultz-Sous-Forest, France or a research model of wastewater containing Chloride flowing in groundwater through fractured rocks [24].


ANN (Artificial Neural Network) method: is a new method and has received much attention recently in the oil and gas industry. This method can be used in areas with medium to good quality seismic data, in areas in the exploration and development stages and with a limited number of wells. Some typical mines that have applied the ANN method are Bach Ho and Hai Su Den. For this method, the user will train the method with input data from the well. The method mainly uses seismic data for interpolation. Therefore, for mines that are in the undeveloped stage, in the exploration and appraisal stage, with good seismic data, the ANN method will prove to be more effective because this method can combine seismic data and automatically interpolate according to the actual guidance documents that are entered. Therefore, the method can avoid errors due to the subjectivity of the interpreter. The most prominent feature of this method is the ability to combine weighted input data. This weight is determined during the training of attributes using well documentation.

ANN allows to combine a series of seismic attributes trained on well data, operating on the basis of applying nonlinear algorithms. Each attribute will be multiplied by the corresponding weight and summed to calculate the porosity.

To increase the accuracy of the model, geostatistical algorithms (Co-Kriging) are used based on the combination of some other well documents such as FMI, PLT, and geological and tectonic information of the study area.

Co-Kriging is an interpolation method based on a number of input documents. The method uses statistical characteristics to control the interpolation method. Based on the characteristics of both Co-Kriging and ANN methods, if these two methods are combined, a combination of data will be created: seismic data, well geophysical data and geological - tectonic information in the study area. Therefore, the outstanding advantage of the proposed method is to combine all the available data into the porosity model.


Currently, the construction of porosity model for cracked foundation by combining ANN and Co-Kriging method has not received attention and been widely applied in Vietnam.

2.1.2. Mechanism of fracture formation in granitoid basement rocks

Basement is a general term used to describe all rocks that formed before the formation of the basin and form the bottom of the basin. Basement may extend beyond the basin and be exposed on the ground.

The Cuu Long Basin began to form in the mid-Eocene period, due to the spreading process developing on the continental crust in the east of the Da Lat zone [16]. The basement rocks of the basin are sedimentary rocks, metamorphosed sediments, intrusive and eruptive magmatic rocks before the Cenozoic as seen in the Da Lat zone. In these basement rock formations, intrusive magmatic rocks are a very special reservoir rock, known as "fractured granitoid basement reservoir" (Figure 2.3) . The petrological composition of granitoids includes granite, monzonite, granodiorite, quartz diorite, monzodiorite, diorite, gabbrodiorite... They are divided into magma complexes with ages from Late Triassic to Late Jurassic - Cretaceous: 1) Hon Khoai Complex (183-208 Ma) of Late Triassic age; 2) Dinh Quan complex, Deo Ca complex of Late Jurassic-Cretaceous age, and Ankroet complex (Ca Na) (100-130 million years ago) [9].

Figure 2.3. Model of oil and gas trap in Cuu Long basin fractured basement: (1) fractured basement reservoir rock;

(2) Clay group D – blocking and generating layers; (3) Sandstone-bearing rocks.


Figure 2.4. Classification of basement rocks according to geological and lithological units.


Figure 2.5. Classification of granitoid rocks from some wells in the Cuu Long basin (according to the classification of Streckeisen, 1976).


Cracks forming in hard rocks can be caused by many different reasons, or a combination of some reasons. For cracks and cavities in granite basement rocks in Bach Ho and Rong mines, researchers Areshev EG, Tran Le Dong and others (1992), Ngo Xuan Vinh (1999), Phan Trung Dien (2000), Pham Anh Tuan (2001), Trinh Xuan Cuong (2013)..., all have the common opinion that there are the following main reasons [13, 17, 18, 21, 22]:

- Cooling and volume shrinkage of magma mass: Due to the magma mass intruding into contact with the surrounding rock. Accompanying that process is the volume shrinkage of magma forming primary cracks including the following typical types: 1) Horizontal cracks (Q) perpendicular to the contact surface between the magma mass and the surrounding rock, and at the same time perpendicular to the orientation of minerals in the contact zone. They are often relatively straight, with a flat surface. In the belts, cracks Q spread out like fan ribs, with a large opening; 2) Vertical cracks (S) perpendicular to (Q). Vertical cracks are often short, with uneven surfaces, very steep or vertical and are closed cracks; 3) Layered cracks (L) or gentle cracks are often parallel to the contact surface; 4) Diagonal cracks (P) intersect and receive (Q) as a bisector, often developing unevenly.

Figure 2.6. Primary fracture patterns of intrusive magmatic rocks (source: block diagram of Cloos.E).

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