3.1 Oil content and molecular composition of shale fractions
3.1.1 Characteristics of oil content and molecular composition of fractions from frozen shale samples in situ
Characteristics of the oil content and molecular composition of frozen (liquid nitrogen) shale samples collected by coring at the drilling site (Fig. 1). The oil contents of the S1-0, S1-1, S1-2, S1-3 and S1-4 fractions of 1620.43 m deep shale samples from well cha21 are 1.25 mg/g, 0.44 mg/g, 0.33 mg/g, 0.31 mg/g and 0.38 mg/g, respectively. The main molecular compositions of the samples are nC1-nC9, nC1-nC15, nC12-nC20, nC15-nC22, and nC18-nC26. The five temperature fractions were mainly gas, gasoline, kerosene, diesel oil and heavy oil (Department of Organic Chemistry, 1984). The contents of the samples were 46%, 16%, 12%, 11%, and 15%, and the main components were gas and gasoline. The parent material type is II1, and the maturity Ro is 0.80%, which is in the early stage of maturity. This kind of sample has the highest gas fraction.
3.1.2 Characteristics of oil content and molecular composition of fractions from conventional shale samples
Simultaneous analysis of oil content and molecular composition of shale fractions collected by core library show that(Fig. 2),the oil contents of S1-0, S1-1, S1-2, S1-3 and S1-4 fractions of 1625.3 m shale samples from well Pu53 are 0 mg/g, 0.04 mg/g, 0.17 mg/g, 0.44 mg/g and 1.42 mg/g. The main distribution of its molecular composition was not detected; nC11-nC15 (n-alkane degradation), nC13-nC17 (n-alkane degradation), nC15-nC20 (n-alkane degradation) and nC18-nC25 (n-alkane degradation) accounted for 0%, 2%, 8%, 21% and 69%, respectively. The n-alkanes of gasoline, kerosene and diesel oil were biodegraded; they were mainly composed of heavy oil, and no biodegradation occurred. The type of hydrocarbon parent material is II1, and the maturity Ro is 0.83%, which is in the early mature stage of hydrocarbon expulsion. The oil contents of the S1-0, S1-1, S1-2, S1-3 and S1-4 fractions of the 2135.78 m shale sample in well syy2 are 0 mg/g, 0.83 mg/g, 1.19 mg/g, 0.93 mg/g and 0.66 mg/g, respectively. The main distribution of its molecular composition was not detected. The contents of nC13-nC20, nC17-nC23, nC20-nC26 and nC23-nC29 accounted for 0%, 23%, 33%, 26% and 18%, respectively. It is mainly composed of kerosene, diesel oil and gasoline. The type of hydrocarbon parent material is II1, and the maturity Ro is 1.19% in the mature stage. The oil contents of the S1-0, S1-1, S1-2, S1-3 and S1-4 fractions of 2557.36 m shale samples from well Gy1 are 0.06 mg/g, 0.63 mg/g, 1.17 mg/g, 1.27 mg/g and 0.16 mg/g, respectively. Its molecular composition is mainly distributed in nC10-nC13, nC11-nC16, nC12-nC18, nC13-nC21, nC15-nC20, and the contents were 2%, 19%, 35%, 39% and 5%, respectively. It is mainly composed of diesel oil, the type of hydrocarbon parent material is type I, and the maturity Ro is 1.67%, which is in the high maturity stage. The oil content and molecular composition of the fractions are different in different well shale samples of the first member of the Qingshankou Formation. This reflects the differences in shale with different parent material types and maturities.
3.2 Evaluation of oil bearing and fluidity of shale reservoir
3.2.1 Evaluation of oil bearing property of shale reservoir
The larger the oil content of the shale fraction and its value, the better the oil-bearing property of the reservoir under the same analytical conditions. The oil contents of shale in wells cha21, Pu53, Gy 1 and syy2 are 1.33 mg/g, 2.09 mg/g, 3.29 mg/g and 3.61 mg/g, respectively. In terms of oil content, syy2 and Gy 1 are the best, Pu53 is the second, and cha21 is the last (Table 1). However, it should be noted that although the oil content of well Pu53 is higher than that of well cha21, the components of gasoline, kerosene and diesel oil in the first member of the Qingshankou Formation in well Pu53 are biodegraded, the degradation of n-alkanes is serious, the crude oil becomes thicker and the oil quality is obviously worse.
Table 1 Characteristics of fraction content and molecular composition of shale with different maturity
Well name
|
Deep
(m)
|
TOC
(%)
|
Ro
(%)
|
S1-0(%, mg/g, Carbon number range )
|
S1-1(%, mg/g, Carbon number range )
|
S1-2(%, mg/g, Carbon number range )
|
S1-3(%, mg/g, Carbon number range )
|
S1-4(%, mg/g, Carbon number range )
|
Oil content(%, mg/g, Carbon number range )
|
(S1-1+ S1-2+ S1-3)/ S1-4
|
Cha21
(frozen)
|
1620.43
|
2.62
|
0.80
|
46,1.25,
nC1~nC11
|
16,0.44,
nC10~nC15
|
12,0.33,
nC12~nC20
|
11,0.31,
nC15~nC22
|
15,0.38
nC18~nC26
|
100,2.71, nC1~nC26
|
2.60
|
Cha21
|
1620.43
|
2.62
|
0.80
|
/
|
29,0.39,
nC10~nC15
|
26,0.34,
nC12~nC20
|
24,0.32,
nC15~nC22
|
21,0.28,
nC18~nC26
|
100,1.33, nC10~nC26
|
3.76
|
Pu53
|
1625.30
|
2.86
|
0.83
|
/
|
2,0.04,
nC11~nC15
|
8,0.17,
nC13~nC17
|
21,0.44,
nC15~nC20
|
69,1.42,
nC18~nC25
|
100,2.09, nC11~nC25
|
0.45
|
Syy2
|
2135.78
|
2.39
|
1.19
|
/
|
23,0.83,
nC14~nC21
|
33,1.19,
nC16~nC24
|
26,0.93,
nC19~nC26
|
18,0.66,
nC22~nC29
|
100,3.61, nC14~nC29
|
4.56
|
Gy1
|
2557.36
|
4.30
|
1.67
|
2,0.06,
nC10~nC13
|
19,0.63,
nC11~nC16
|
35,1.17,
nC12~nC18
|
39,1.27,
nC13~nC21
|
5,0.16,
nC15~nC20
|
100,3.29, nC10~nC21
|
18.60
|
Gy1
(frozen)
|
2570.65
|
2.33
|
1.67
|
31,1.46,
nC7~nC12
|
33,1.53,
nC10~nC16
|
18,0.85,
nC11~nC18
|
11,0.50,
nC14~nC20
|
7,0.31,
nC17~nC23
|
100,4.65, nC7~nC23
|
8.86
|
3.2.2 Fluidity evaluation of shale reservoir
Under the same or similar reservoir physical properties, the larger the light component proportion and the smaller the heavy component proportion of shale oil, the better the reservoir crude oil fluidity. The gas fraction in the conventional shale sample is very easy to lose; therefore, we adopt (s1-1 + S1-2 + S1-3)/s1-4, that is, (steam + coal + diesel)/heavy oil ratio (light weight ratio parameter). This parameter is relatively less affected by light hydrocarbon loss. We use this parameter to evaluate crude oil fluidity in shale reservoirs (Table 1). The light weight ratios of wells cha21, Pu53, syy2 and Gy1 were 3.76, 0.45, 4.56 and 18.6, the order of liquidity was Gy1 > syy2 > cha21 > Pu53, and their maturity Ro values were 0.80%, 0.83%, 1.19% and 1.67%, respectively. Therefore, the higher the maturity is, the better the fluidity of shale oil. However, the crude oil maturity RO of well Pu53 is slightly higher than that of well cha21; because of the degradation of crude oil in well Pu53, the fluidity of well Pu53 worsens. According to the results of exploration and testing, the fracturing of Cha21, Syy2 and Gy1 shale can obtain industrial oil flow, so (steam + coal + diesel)/heavy oil ≥ 3 is taken as the evaluation boundary value of liquidity desserts.