About 80% of the oil and gas in the Middle East is produced from carbonate reservoirs^[1,2,3,4]. The Cretaceous Mishrif Formation is the most important reservoir in the Middle East^[5,6,7], which is developed widely in Iraq, Kuwait, Saudi Arabia, and the United Arab Emirates^[8]. The carbonate reservoirs of Mishrif Formation developed in central and southern Iraq have proven reserves accounting for about 40% of the Cretaceous oil reserves and nearly 30% of the total oil reserves in Iraq^[9].

The H oilfield, located in southeastern Iraq, is currently at the initial stage of development, where the Mishrif Formation is major target of Cretaceous carbonate. Many achievements have been obtained in the previous studies on sedimentation and reservoirs of Mishrif Formation. Some researchers thought that the tectonic activity was weak during the deposition of Mishrif Formation^[10] in open platform and platform margin environments^{[6, 11-13]}; and the formation and reformation of reservoir space were mainly controlled by the depositional environment, penecontemporaneous dissolution and late supergene karstification^{[6, 14-20]}. After comprehensive study of Mishrif Formation in the H oilfield, we reached the following findings: (1) The depositional environment should mainly be open platform, where there may be platform margin deposits but not in a large scale^[21]. (2) The karst type is the facies-controlled eogenetic karstification, which is characterized by the combination of penecontemporaneous dissolution and exposed dissolution in shallow buried period^[22]. The conceptual model of karst development has been established, and three zones are identified according to the characteristics of dissolution-filling karstification intensity in cores from high to low, namely, the tight basement zone, cavernous dissolution zone and groove (pore) filling zone^[22]. (3) The reservoirs are mainly composed of karst formations, grainstone, packstone, and wackestone, etc., with the reservoir space dominated by intrafossil pores, intragranular pores (moldic pores), intergranular pores, inter-filling micropores, and framework pores^[22,23]. (4) The formation of reservoir space and pore throat structure mainly depends on the sedimentary microfacies and eogenetic karstification^[22,23]. Further studies showed the coring intervals of Mishrif Formation in the H oilfield have big differences in oil-bearing property and strong heterogeneity. Even though physical properties and oiliness are in positive correlation in general, there are exceptional reservoir section with good physical properties but no oil, indicating the law of oil-bearing property needs to be explored in depth. It is of significant reference value to the prediction of effective reservoirs in the future. However, no study on this subject has been reported yet.

Based on a large amount of basic experimental data and previous achievements, the major control factors of reservoir oil-bearing property of Mishrif Formation in the H oilfield were preliminary studied and analyzed, in the hope to guide the later practical exploration and development of the oilfield, and provide directions and experience in the oiliness analysis of the carbonate reservoir in Mishrif Formation of H oilfield.

The H oilfield is located in Misan Province, southeast of Iraq, and is 400 km from Baghdad, the capital of Iraq. The study area covers an area of about 288 km². Tectonically, it is the unstable continental shelf in the north of the Arabian Plate on the northern margin of Gondwanaland, in the south of the eastern unstable continental shelf, Mesopotamian and Tigris. It is a third-order tectonic unit with the largest burial depth, thickest deposits and relatively stable structure in the Mesopotamian Basin^[24,25], with platform deposits developed (Fig. 1), and an anticline extending from northwest to southeast gently. The Mishrif Formation deposited in the Mid-Late Cretaceous Period, is mainly composed of grainstone, packstone, wackestone, and shell floating rocks^[24]. It comes in bands in southeast strike at the border between Iraq and Iran and in Basra^{[26,27,28,29]}. The overlying Khasib Formation consists of grey-brown chalky limestone and microcrystalline clastic limestone, and is in unconformable contact with the Mishrif Formation. The Mishrif Formation is in conformable contact with the underlying Rumaila Formation composed of largely yellow brown-grayish white porous limestone. As the most important oil-producing layer, the Mishrif Formation is up to 400 m thick, and is further subdivided into 18 layers (Fig. 2). Its top is a regional unconformity due to Late Cretaceous Laramide Orogeny^[7].

This work is mainly based on the data of Wells M316, N137, N195, Y115 and Y161, in which the coring lengths of Mishrif Formation are 286.92 m, 107.50 m, 77.00 m, 81.00 m and 45.00 m, respectively. The core photos are complete, in which the oil-bearing features are clear and easily observable. As the coring intervals include the MC2-1 to MA1 sub-layers, the data of all the wells combined can reflect the overall deposition, reservoir development and oiliness distribution of these intervals (Fig. 2). At an average sampling frequency of 1/m, a total of 603 thin slices were taken, including 286, 105, 87, 80 and 45 from the cores of the five wells respectively. All thin slices were provided by the Research Institute of Petroleum Exploration Development, and their observation and analysis were completed by the Sedimentary Geology Research Center of Southwest Petroleum University, Key Laboratory of Carbonate Reservoirs, CNPC. On the basis of thin slice analysis, the microscopic constitution of thin slices was studied, with focus on distinguishing reservoir space types, dissolution features and oiliness. Moreover, physical property data of more than 1800 samples and mercury injection data of 920 samples can provide data support for discussion on the main factors causing reservoir oiliness difference.

Based on the basic data acquired and previous achievements, the reservoir oiliness is further analyzed, and the following results are obtained.

2.2.1. Division of oil-bearing grade

Through observation and analysis, the cores of Mishrif Formation collected from 5 coring wells in the H oilfield can be divided into 4 oil-bearing grades, oil rich (oil saturated), oil immersed, oil spot, and oil trace according to the Standard SYT5364-89^[31] (Fig. 3). The oil rich grade means the oil-bearing area is more than 70% of the total rock area. For example, the karst rock from 3 042.55 m of Well N137 has a characteristically high content of oil. Dark brown, the rock is full of and evenly saturated with oil, in striking contrast to the oil-free bioclasts (Fig. 3a). The oil immersed grade means the oil-bearing area is 40%-70% of the total rock area. For example, the bioclastic packstone from 3 131.50 m of Well N137, with grooves of irregularly shape filled with carbonate muds and sands. It is dark brown, and has overall high oil content, but the base rock is grayish white and oil-free (Fig. 3b). The oil spot grade denotes the oil-bearing area is 5%-40% of the total rock area. For example, in the bioclastic wackestone from 3034.35 m of Well N195, are organism burrows in spots and strips, based on which, grooves and caves are formed. The fillings are relatively oil-rich and light brown, but most of the original rock is oil-free (Fig. 3c). The oil trace grade means the oil-bearing area is less than 5% of the total rock area. For example, in the bioclastic wackestone from 3086.70 m of Well M316, the oil-bearing parts are scattered organism burrows with light brown fillings, and the original rock is grayish white and fairly tight (Fig. 3d).

2.2.2. Oiliness of coring intervals

Taking Well M316 with the most abundant coring data as an example, the composite columnar section of oiliness shown as Fig. 4 was compiled according to the detailed description and analysis of cores. The Mishrif Formation includes multiple cycles that are upward shallowing cycles dominated by descending semi-cycles, with wackestone and micrite limestone of interbank subfacies and open sea subfacies at the bottom, and wackestone and grainstone of platform low-energy beach subfacies and platform margin high-energy beach subfacies at the top. Karstification is very common throughout the whole interval, there are karst caves and grooves in chaotic occurrence in almost every single cycle. In the relatively high porosity and permeability layers dominated by grainstone and packstone, karst caves and low-angle grooves are the major reservoir space and are filled by plastic breccia, carbonate mud and sand, and clasts. In addition, there are a large number of cavernous pores with generally favorable oiliness. In the relatively low porosity and permeability layers dominated by wackestone and micrite limestone are a large number of organism burrows, and high-angle and low-angle grooves develop based on them; there are also cavernous dissolution pores in some parts, with oil appearing in spots and patches on the whole.

The vertical oiliness of Mishrif Formation was further studied based on the interpretation of typical sedimentary-oil- bearing characteristics at specific depth. In the depth interval from 2 898 m to 2 906 m in Well M316, four upward shallowing cycles were identified. Each cycle consists of interbank wackestone in the lower part and platform packstone in the upper part. The packstone and wackestone have clearly different dissolution and oil-bearing characteristics. In the packstone are many irregular grooves, and a large amount of plastic karst breccias or brecciated original rocks, this rock is abundant in oil, showing oil-rich to oil-saturated characteristics. The wackestone contain a large number of organism burrows, and high-angle grooves or caves created based on the organism burrows. It generally is oil-immersed or oil-spot grade (Fig. 5).

2.3.1. Oil-bearing grade distribution in major types of reservoirs

After analyzing the distribution frequency of oil-bearing grades in major types of reservoirs in coring intervals of Wells N137, N195, Y115, Y161 and M316, some regularity was found (Fig. 6). The wackestone is mostly oil spot grade, with a occurrence frequency of 87.3%. The occurrence frequency of oil rich, oil immersed and oil trace grades are 1.4%, 2.6% and 8.7%, respectively. In the packstone, the occurrence frequency of oil spot decreases significantly to 44.6%, while that of oil immersed and oil rich increase remarkably to 35.9% and 19.5%. In the grainstone, the frequency of oil spot is the lowest, 22%, while that of oil immersed and oil rich grades are 70% and 8% respectively. As the reservoir rocks vary from low-energy rocks to high-energy rocks, the oil-bearing grade of cores also change from oil spot, oil trace to oil immersed, oil rich, showing obvious facies-controlled characteristic.

2.3.2. Occurrence frequency of dissolution-filling zones of different oil-bearing grades

According to the occurrence frequency of karst zone at sample points of different oil-bearing grades on cores, in the oil-saturated and oil-rich cores, the karst grooves (caves) filled zone takes dominance, with an occurrence frequency of 83.7%, followed by the cavernous dissolution zone with a frequency of 16.3%. In the oil-immersed grade, the occurrence frequency of karst grooves (caves) filled zone is 68%, that of the cavernous dissolution zone is 28.7%, and that of the tight basement zone is 3.3%. In the oil spot grade, the occurrence frequency of karst grooves (caves) development zone further decreases to about 38.2%, frequency of cavernous dissolution zone and tight basement zone are 47% and 14.8%. In the oil trace grade, the occurrence frequency of tight basement zone is close to 100%. Therefore, with the increase of karstification strength, the oil-bearing grade of core gradually increases (Fig. 7).

2.3.3. Physical properties of dissolution-filling zones in different oil-bearing grades

The statistics of physical properties of karst zones where sample points locate on coring intervals in different oil-bearing grades show: The grooves (caves) filled zone in the oil- saturated and oil-rich grade cores has the best physical properties, with an average porosity of 21.2% and permeability of 34×10³ μm², followed by the cavernous dissolution zone, with an average porosity of 14.5% and permeability of 1.3×10³ μm². The groove (cave) filled zone in the oil-immersed grade cores has the best physical properties, with an average porosity of 21.9% and permeability of 25.4×10³ μm², followed by the cavernous dissolution zone, with an average porosity of 21.5% and permeability of 8.7×10³ μm². The groove (cave) filled zone in the oil-immersed grade cores also has the best physical properties, with an average porosity of 22.8% and permeability of 15.5×10³ μm². The cores of oil trace grade are mainly discovered in the tight basement zone with poor physical properties. In conclusion, in cores of different oil-bearing grades, the groove (cave) filled zone always has the best physical properties, followed by the cavernous dissolution zone, and the tight basement zone has the poorest physical properties (Fig. 8).

2.3.4. Pore-throat structure of dissolution-filling zones of different oil-bearing grades

In this study, five typical types of mercury intrusion curves were identified, with pore-throat structure gradually worsening from type I to type V. According to the distribution frequency of typical mercury injection curves in karst zones in different oil-bearing grades (Table 1), the oil-bearing groove (cave) filled zone in the oil-saturated and oil-rich cores is dominated by type I and type II curves, while the cavernous dissolution zone is dominated by type I, type II and type III curves, with favorable pore-throat structure. In the oil-immersed cores, the oil-bearing groove (cave) filled zone is dominated by type I and type II curves, the cavernous dissolution zone is dominated by type I, type II and type III curves, and the oil-free tight basement zone is dominated by type III, type IV and type V curves. In the oil spot cores, the oil-bearing groove (cave) filled zone is dominated by type I, type II and type V, the cavernous dissolution zone is dominated by type I and type III curves, and the oil-free tight basement zone is dominated by type IV and V curves. In the oil trace cores, the oil-bearing groove (cave) filled zone and cavernous dissolution zone are characterized by type I curve, while the oil-free tight basement zone is mainly characterized by type IV and V curves.

3.1.1. Determination of depositional microenvironment and eogenetic karstification on the distribution of macroscopic reservoir oiliness

According to the above statistics, the sedimentary microen-vironment is the basic factor determining the macro distribution of oil-bearing property in reservoirs of Mishrif Formation in H oilfield.

In the grainstone, packstone and other relatively high-energy deposits, the rich primary intergranular pores provide favorable material basis for later reformation by karst water or oil and gas filling, so beach deposits have a higher oil content than low-energy deposits such as interbank and open sea. The eogenetic karstification also controls the reservoir oiliness strongly. Under the influence of facies-controlled keogenetic karstification, areas with different karst development intensity, including the groove (cave) filled zone, cavernous dissolution zone and tight basement zone, are formed. In intervals with well-developed groove (cave) filled zone and cavernous dissolution zone, the karstification reformed zone has better physical properties and pore-throat structure, is filled with oil evenly and often oil saturated and oil rich or oil immersed. In contrast, the area reformed by karstification limitedly is always dominated by tight basement zone, but in areas reformed by karstification, the groove (cave) filled zone and cavernous dissolution zone still have fairly good physical properties and pore-throat structure and higher oil content than the basement zone, with uneven oil-bearing distribution, so they are often oil spot and oil trace grades. In other words, the material basis of reservoir and karst development intensity are the major factors determining the macroscopic distribution of reservoir oiliness.

3.1.2. Improvement of pore-throat structure is the decisive factor of the micro oil-bearing difference of reservoir

Based on observation of cores and thin sections at sample points and statistics of basic data, it is found that a great difference in oiliness would occur in the case of similar lithology and physical properties. For example, the samples taken from 3060.38 m and 3061.38 m in Well N137 are both bioclastic wackestone with a porosity of about 11%-13% and permeability of (2-3)×10³ μm², with little difference. But the oiliness observation shows the core at 3 060.38 m is oil rich and oil saturated, and the core at 3 061.38 m is only oil spot grade. Further microscopic observation reveals dissolution is more intense at 3 060.38 m, giving rise to a largenumber of intragranular pores, moldic pore, intergranular pores, so the overall pore-throat structure is better (Fig. 9a). At 3061.38 m, the pores include a small number of intragranular dissolution pores and a large number of matrix micropores few microscopically visible, and the measured porosity reaches 12.9%, which further reflect the poor-pore throat structure (Fig. 9b). It is also confirmed by the mercury intrusion experiment results. All the pore-throat structure parameters of this sample point are worse than those at 3 060.38 m (Table 2), which indicates the pore-throat structure is the decisive factor affecting oil-bearing property microscopically. Although the later cementation would have a destructive effect on pores and throats formed previously, as the Mishrif Formation in H oilfield features strong dissolution and weak cementation, cementation has less effect on its pore-throat structure^[32]. Therefore, the eogenetic karstification is undoubtedly the most critical factor for the improvement of pore-throat structure on the basis of primary deposition.

Based on the above analysis, it is concluded that the formation of reservoir space in Mishrif Formation of H oilfield is mainly controlled by sedimentary microfacies and eogenetic karstification; whereas the reservoir quality and pore-throat structure directly influence the oiliness of reservoir, resulting in the oil-bearing grade differences and uneven distribution of oil. Thus, the genetic model of reservoir development and oil-bearing difference is established (Fig. 10).

Specifically, at the end of cyclic short-term deposition of several beaches in the open platform, the relatively high part of beaches exposed for a short period due to the fall of relative sea level. Therefore, packstone, grainstone and other relatively high-energy rocks were subjected to early selective dissolution, forming cavernous dissolution zone; while the low interbank sea and open sea didn’t have exposure conditions, but were suitable habitat for burrowing organisms, therefore, organism burrows are common in wackestone, micrite limestone and other low-energy rocks (Fig. 10a). In the subsequent mid-term regression, the previously deposited shallow buried strata exposed again, and grainstone and packstone were reformed again on the basis of the previous dissolution, forming caves and grooves that were filled with plastic breccia, loose carbonate sediments and biological debris later. Moreover, the organism burrows and fractures in the wackestone and other low-energy rocks acting as channels of karst water were reformed by karstification and enlarged due to dissolution, even connecting the upper and lower beaches with higher porosity and permeability in local parts. Some parts with relatively higher porosity and permeability also ended with the cavernous feature as a result of selective dissolution (Fig. 10b). With another mid-term transgression-regression cycle taking place, the above processes occurred repeatedly, multiple mid-term cycles overlapped, finally giving rise to the frequent occurrence of eogenetic karstification vertically (Fig. 10c, 10d). In the burial stage, when the crude oil entered into the reservoirs after secondary migration, as the Mishrif Formation was well connected due to eogenetic karstification, the whole formation was oil-bearing overall. But the difference of pore-throat structure in different rocks caused by karstification led to the different oil-bearing grades. With more groove (cave) filled zone and cavernous dissolution zone and favorable pore-throat structure, the grainstone and packstone are often higher in oil-bearing grade, appearing as oil rich-oil saturated and oil immersed. In the wackestone and micrite limestone, due to the general poor pore-throat structure of basement, crude oil only charged into high-angle grooves and a few cavernous dissolution zones formed by burrows, generally forming oil spots, oil traces or even oil-free parts (Fig. 10e).

This study preliminarily clarified that the favorable sedimentary microfacies and eogenetic karstification are two key factors determining the reservoir oiliness of Mishrif Formation in H oilfield. Both of them are indispensable. Compared with paleogeomorphologically lower beaches, the higher ones have better original pore-throat structure and homogeneity after being washed more fully and sorted better, which is more favorable for the full combination of later eogenetic karstification. Thus, in the later study of the plane distribution of effective reservoirs, the area where high beaches overlap with the present structural high parts should be the area with most developed effective reservoirs, which is classified as the type I area; the area where low beaches overlap with the present structural high parts is classified as the type II area of effective reservoir.

The coring intervals of Mishrif Formation have strong oiliness heterogeneity, and can be divided into 4 grades, oil rich, oil immersed, oil spot, and oil trace. The general oiliness is high.

The sedimentary microenvironment is the basis of the formation of original reservoir space. In a relatively high-energy beach, more high-quality primary pore throats would be formed due to sufficient washing, as a result, it often has higher oil content than low-energy non-beach.

The improvement of local pore-throat structure caused by eogenetic karstification is the decisive factor leading to the micro oil-bearing difference of reservoir. In cores of different oil-bearing grades, the higher the karstification intensity, the better the pore-throat structure, and the higher the oil content will be, which directly lead to the oiliness heterogeneity on core scale.

The area where the present structurally high part superimposes with relatively high-energy beaches has the greatest oil potential and is where effective reservoir is most developed.