When drilling has not yet reached the oil-bearing interval, the sand samples and cores of the overlying strata are picked and ground into rock slices, observed and recorded under a microscope, and a series of data such as the types, contents, shapes, sizes, interrelationships, distribution intervals, continuity and metasomatism of special minerals are obtained, which are compared with the indexes of special minerals in the study area, so as to make a comparison with the underlying areas that have not yet been drilled.
this method is beneficial to detect the oil-gas bearing property of the underlying strata in time, guide the smooth drilling work, and especially provide a basis for the decision whether to deepen drilling or finish drilling ahead of schedule. The following are some application examples in western Qaidam oil and gas area.
1. Well Qidong-1 successfully predicted oil production
Well Qidong-1 occurred in the interval of 2,132 ~ 2,838 m, and the comprehensive characteristics of the special minerals reflected that its index met the index standard of Class I special minerals in western Qaidam. The tested oil production is 41.5m3/d in the interval of 2793.～2819.5m, and 6.5m3/d in the interval of 2958.4～2961.5m.
this well is the first exploratory well to analyze and predict the oil-gas bearing property of the well being drilled by using the special mineral method. During the drilling process, it was considered as a gas well with high oil production after the analysis and prediction by using the special mineral method before the midway test, which was later confirmed by the midway test.
from 1962 to 2852 m, 52 samples of special minerals were analyzed in this well. The samples were mainly fine-medium sandstone and siltstone, with a small amount of micrite limestone and argillaceous micrite limestone.
from 1962～2125m, there were 21 observed samples of special minerals, mainly siltstone, micrite limestone, dolomitic limestone, oolitic limestone, fine sandstone and medium sandstone. Except for a small amount of magnetite or limonite in 3 samples, no special minerals were found in the other 18 samples. None of all the samples contains special minerals with obvious indicative significance, and the samples with special minerals only account for 14.3% of the observed samples, and the content is below .2%, while the samples without special minerals are 18, accounting for 85.7%. The three samples with special minerals did not contain the characteristics of special minerals with obvious indicative significance.
from 16 samples of 2132 ~ 2284 m, the well interval with obvious indicating significance for the first time is 2132m, which is nearly 7m away from the oil-gas producing interval. The lithology here is siltstone, pyrite is 3%, most of them are irregular, some are cemented, and the diameter of cemented range is about .5 mm. In this interval, there are only two samples of special minerals with obvious indicating significance, accounting for only 12.5% of the observed samples, while the samples without special minerals account for 87.5%. In addition to siltstone at 2132m, another sample of special minerals with obvious indicative significance is at 2241m m. This sample is fine-medium sandstone, with pyrite accounting for 2.5%, which is massive, irregular and annular.
from 233～2838m, the span is about 54m. Contrary to the previous paragraph, only one of the 14 observed samples does not contain special minerals, accounting for only 7.2%, while 82.8% of the samples contain special minerals pyrite with obvious indicative significance, with the lowest content of .8% and the highest content of 5%, generally 1.5% ~ 4%. The aggregate shape is disseminated granular, spherical aggregate, massive, annular granular and filled with cracks. Figure 9-1).
ii. evaluation of characteristic mineral indexes of well yue 74
the characteristic mineral indexes of well yue 74 in the interval from 1674 to 3144 m belong to class iii, and only one layer of oil-stained argillaceous siltstone and two gas logging anomalies were found in the Miocene.
the analysis interval of special minerals in Well Yue 74 is 1674 ~ 3144 m, with 35 samples. Its lithology is mainly fine sandstone and siltstone, and some limestone and granite. At 1932m, special minerals were first seen in siltstone, with magnetite accounting for .15% and pyrite < .1%. The characteristic minerals in the interval from 1932 to 35 m show the following six characteristics: ① The pyrite content is the lowest < .1%, the highest is 1.5%, and generally < .5%. ② Magnetite is rare, the highest is .15%, generally less than .1%. ③ No limonite was found. ④ The shape of aggregates is mostly blocky, irregular, individual ring-shaped and carbonate-associated, and there are no oversize blocks, filled cracks, ring-contact belts, spherical aggregates, automorphic crystals enriched and automorphic crystals embedded, and there are no automorphic-semi-automorphic single crystals. ⑤ Only a few aggregates reach .15mm, and most of them are less than .5mm.. ⑥ No straw yellow halo was found around the special minerals.
Table 9-2 Statistical Table of Characteristic Mineral Analysis of Well Qidong 1
Continued Table
Figure 9-1 Comprehensive Histogram of Characteristic Mineral Analysis of Well Qidong 1
Compared with the contents of various characteristic mineral indicators, it can be seen that the characteristic mineral indicators indicating the oil-bearing property of Well Yue 74 belong to three categories.
Well Yue 74 only has one layer of oil-trace argillaceous siltstone and two layers of gas logging anomalies (one of which has bubbles on the groove surface) in E23, and the electrical logging interpretation of the whole well does not explain the layers related to oil and gas. It is proved that the evaluation of special mineral indexes indicating the oil and gas bearing property of this well is accurate.
However, it is worth noting that four samples containing pyrite, a special mineral, were seen from 3952 to 34 m, and the analysis shows that it may indicate that there is a certain possibility of oil in the underlying bedrock-granite (Table 9-3; Figure 9-2).
Table 9-3 Statistical Table of Special Mineral Analysis of Well Yue 74
Figure 9-2 Comprehensive Histogram of Special Mineral Analysis of Well Yue 74
III. Evaluation of Special Mineral Index of Well Shi 32
The special mineral distribution interval of Well Shi 32 is 2,318 ~ 4,18 m, including 2,464.5 ~ 3,977 m interval (which contains many gypsum layers).
in the interval of 2318～418m, 6 samples of special minerals were observed, and the lithology was mainly micrite dolomite, micrite limestone, mudstone, individual siltstone and sandstone. Of the 6 samples from 2318m to 418m, 58 samples showed that special minerals were distributed in different degrees. For example, at 252m, micrite-bearing dolomite and magnetite are semi-filled in four fractures of vertical bedding; At 2642m, the proportion of magnetite in micrite limestone is 4.%, which is only locally concentrated in the silt, mainly in blocks; At 285m, the content of pyrite in siltstone is 3.%, which is mostly disseminated granular and massive, and a small part is irregular and individual annular granular. Maximum .15mm; At 35m, pyrite is 1.8% in mudstone, which is in the form of pellet aggregation and lump, and the pellets are between .5 and .2 mm; At 356m, in the silty limestone, magnetite is mostly enriched in silt, which is disseminated in granular, irregular and massive form, with the maximum of .25mm; At 396m, in the mudstone, pyrite is 1.3% and magnetite is .4%, and magnetite is distributed intermittently around a salt granule. At 46m, in argillaceous dolomite, magnetite is 4.5%, which is distributed in a strip shape intermittently along the micro-bedding plane to form bedding; At 415m, in the argillaceous dolomite, a large number of local magnetite are abnormally distributed in the silt-bearing part, which is cemented, and there is a pyrite block wrapped in it. At 4171m, semi-authigenic pyrite, with a size of .14mm, is locally seen in mudstone, which is wrapped with metasomatic residual debris particles (Table 9-4; Figure 9-3).
Table 9-4 Statistical Table of Characteristic Minerals Analysis of Well Shi 32
Continued Table
Figure 9-3 Comprehensive Histogram of Characteristic Minerals Analysis of Well Shi 32
The characteristic minerals of Well Shi 32 reflect the following eight characteristics: ① The characteristic minerals are continuously distributed in the interval of more than 1,7 meters from their appearance to the producing interval. ② The special minerals are mainly pyrite and magnetite, while pyrrhotite and limonite are rare. ③ The highest content of pyrite and magnetite is 9.2%, generally between 1% and 5%. ④ The maximum number of blocks is mm, generally less than .1 mm.. ⑤ The aggregate forms are blocky, irregular, disseminated, spherulitic, annular, cemented, banded, bedded, and filled with cracks, and lack of automorphic crystals, automorphic crystals embedded, and oversized. ⑥ Individual hemimorphic pyrite crystals are up to .14mm, which are wrapped with metasomatic residual clastic particles. ⑦ Gypsum and rock salt layers in the interval from 2464.5 to 3977m (N1-E23) have no obvious influence on the distribution characteristics of pyrite and magnetite. ⑧ Yellow halo of rice straw around special minerals is rare. Compared with the indexes of various special minerals, it is known that the indexes of oil-bearing special minerals in Well Shi 32 belong to one class. In this well, the oil production was tested in the interval of 424.64 ~ 425.96 m, and the daily oil production of 8mm choke was 249.35m3 and the gas production was 9645m3. It is proved that the evaluation of the special mineral index of oil-bearing property of this well is correct.
IV. Analysis of the index of special minerals in Well Yue Ⅱ146
The content of special minerals in Well Yue Ⅱ146 is slightly lower, but other characteristics are obvious, belonging to Class I index. Electric logging explains that there is one oil layer, two possible oil layers and one oil-water layer, but the daily oil production is only 1.86t in the interval of 3461.61～361.15m (including two possible oil layers). The reasons are as follows: first, the reservoir physical properties become worse; Second, the oil (industrial oil flow) in the oil-bearing layer of the same layer in the lower part migrates to the higher part. If fracturing and other stimulation measures are adopted, the well is expected to obtain industrial oil flow.
1. analysis of special minerals
from 21～3521.1m, 113 samples were analyzed in this well, and 11 samples obtained special mineral information, accounting for 89.3% of the samples. The analysis results show that the special minerals in the interval from 21 to 24 m are mainly magnetite, and the magnetite content is basically less than .1% or around .l% except for some 2.5%(228m), and most of the blocks are less than .5mm except for a few .6 mm.. Only a few samples are pyrite and limonite.
It is worth mentioning that the micrite dolomite at 227m in this section contains 1.2% pyrite, and the aggregate size is generally between .5 and .1 mm, with a maximum of .35mm, with residual pores in the center. A pyrite filling seam is .6mm long and .6mm wide. It reveals the important information that the underlying strata contain oil and gas.
the special minerals below 24 m are mainly pyrite, with a small amount of magnetite and a few limonite. From 26m onwards, the content of special minerals becomes higher (Table 9-5; Figure 9-4), other features also become obvious. Of the 83 samples in the interval of 26 ~ 352 m, 17 (2.5%) have the content of special minerals less than .5%. There are 11 pieces with .5% ~ .1%, accounting for 13.3%; 48 pieces of 1.% ~ 5.%, accounting for 57.8%; There are 7 pieces of > 5%, accounting for 8.4%.
Table 9-5 Statistical Table for Analysis of Special-state Minerals in Well Yue Ⅱ146
Continued Table
Figure 9-4 Comprehensive Analysis Histogram of Special-state Minerals in Well Yue Ⅱ146
Among p>83 samples, 67 special-state minerals are pyrite, accounting for 8.7%; 13 blocks are magnetite, accounting for 15.7%; 1 piece of limonite, accounting for 1.2%; Limonite and magnetite, magnetite and pyrite exist in two blocks, accounting for 2.4%.
among the p>83 samples, 56 samples (accounting for 67.5%) have special mineral blocks (including the range of filling crevices and disseminated bodies) which are mainly > ＞.mm or .5 ~ .1 mm. Among them, 32 samples were found with regular special mineral objects > .1 mm, generally ranging from .1 mm to .2 mm, with a maximum of .35mm; On the samples with abnormal distribution, the maximum is .8mm×5mm; Special mineral filling cracks were found on 6 samples, and the width of the filling cracks was .6mm and the length was 16 mm. * * * Six samples are seen as authigenic-semi-authigenic pyrite single crystals, the minimum is .4mm, and the maximum is .24mm, generally between .5 and .1 mm..
the shape of aggregate is mainly block, followed by disseminated granular, filled with cracks, a small number of filled holes, super-large block, annular contact strip, irregular, annular granular and spherical aggregate. In a few samples, the phenomenon of magnetite containing limonite and the recrystallization of carbonate minerals around pyrite body are also seen, and analcime is also seen in many samples.
2. Characteristics and indicators of special minerals
According to the above data, the following special mineral characteristics of Yue II146 well can be summarized: ① The special minerals are continuously distributed in the 92m long interval from 26～352m. ② The special minerals are mainly pyrite, followed by magnetite, and limonite is rare. ③ The content of special minerals is generally between 1% and 5%. ④ Most of the special mineral aggregates are .5 ~ .1 mm, and .1 ~ .2 mm is common, with a maximum of .35 mm.. Individual abnormal bands reach .8 mm× 5 mm. ⑤ Special mineral filling joints are common, with the widest being .6mm and the longest being 16 mm.. ⑥ Automorphic-semi-automorphic special mineral pyrite single crystals are common, ranging from .4mm to .24mm, generally between .5 and .1 mm.. ⑦ There are some characteristics, such as filling seam, filling hole, ring contact strip, super-large block, disseminated granular, spherical aggregate and irregular.
compared with various special mineral indexes, it is known that the special mineral indexes of Well Yue Ⅱ146 belong to Class I..
3. Confirmatory analysis of characteristic mineral indicators
According to the characteristic mineral indicators of Class I of Well Yue Ⅱ146, the daily oil production of this well should be more than tens of cubic meters. However, the daily oil production of this well is only 1.86m3 when tested in two layers of 3461.6～3464.4m and 3534.6～3537.8m. This output number is within the oil production range indicated by the index of Class II special minerals, and it is very low. It can be seen that there is a category difference between the prediction results of special mineral indexes of Well Yue Ⅱ146 and the formation test results.
It is found that the difference between the predicted oil production and the actual oil production of well Yue Ⅱ146 due to the poor physical properties of the reservoir.
the structural position of E23 in Well Yue Ⅱ146 is slightly higher than that in Well Yue Dong 118, which makes hydrocarbon and non-hydrocarbon gas continuously migrate to Well Yue Ⅱ146 in the higher position along each layer while permeating vertically. Well Yuedong 118 produced 8.92m3 of liquid per day during the oil test from 3,522 to 3,536 m, including 4.74m3 of oil and 4.18m3 of water. Well Yueii 146 with a slightly higher structural position only obtained 1.86m3 of oil per day during the formation test in the corresponding interval. The main reason for the difference is the decrease of porosity. The neutron porosity in the oil test interval of Well Yuedong 118 is 11% ~ 28%, while that of Well Yueii 146. It shows that the high index of special minerals in Yue II146 well is the result of partial migration of hydrocarbon and non-hydrocarbon components in the same oil-bearing interval in the lower structural position to the oil-bearing layer in the higher position. If fracturing and other stimulation measures are taken, the crude oil output of Well Yue Ⅱ146 can be obtained in the above interval, which corresponds to the index of Class Ⅱ special minerals.