Gamma Log Trend Fades in the Choshui Fan-delta , Taiwan

Between 1992and1997, under a project conducted by the Water Re­ sources Bureau, more than 200 well loggings were measured by Taiwan Sugar Corporation Groundwater Center (TSCGC) in the Choshui and Pingtung areas. The purpose of this paper is to evaluate the workability of the log trend method in this hydrogeology investigation project. Previous studies used the Schlumberger formula to calculate the mud content and had a 40% error rate. The reason for this high bias is that mineral assem­ blage, casing, caves and borehole diameter variations shift the natural gamma readings. In the author's experience, borehole diameter variation is the major factor. The gradual shifting greatly reduces the exactness of the Schlumberger formula but isn't a serious problem when using the curve type method. In this research gamma logs in three boreholes of the Choshui fan-delta, Taiwan, are classified to basic log trend facies and compared with the core samples. This study finds eight basic gamma log trend facies and these fades correspond very closely to the mud content or mud thickness. This research suggests that gamma log trends facies are useful to lithology interpretation of TSCGC's data and are valuable for further research into sedimentary environment reconstruction and aquifer geometry prediction. (


INTRODUCTION
Open hole natural gamma logging has been widely applied to calculate shale content for many years (Schlumberger, 1974).In groundwater investigation, the gamma log is used for calculating the mud content.However, previous studies have shown that four out of ten samples had an error in mud content greater than 10%, and have argued that the Schlumberger formula gives the correct rate in only 60% of cases (Table 1; Yuan et al., 1994;Chen, 1997).Serious inaccuracy in using the Schlumberger formula was also found in the Taiwan Sugar Corpora- 2. Calculated mud content by natural gamma from Chen(l997).
3. A mud content error rate larger than 10% will cause misinterpretation.
tion Groundwater Center's log data (TSCGC, 1992(TSCGC, -1997)).In an aquifer, a 10% increase in mud will cause plugging of the porosity in the gravel and sand layer, and it will behave as an aquitard.
Between 1992 and 1997, under a project conducted by the Water Resources Bureau, more than 200 well loggings were measured by TSCGC in the Choshui and Pingtung areas.In the author's experience, borehole diameter variation is the major factor that may shift the natural gamma readings, with minor factors being mineral assemblage, casing, and caves (Figure 1, Table 2).For the purpose of monitoring two aquifer layers, the borehole diameter is usually 40 to 80cm and is bigger in the upper layer than the lower layer.The probe usually goes down and up along the centerline of the well to avoid unconsolidated layers collapsing.Natural gamma readings increase• with depth into the hole as the borehole diameter decreases because as the probe nears the wall of borehole, it receives more gamma rays.Figure 1 shows the trend of a gradual increase in gamma rays with decreasing borehole diameter.The gradual shift greatly reduces the exactness when using the Schlumberger formula but is not a serious problem by curve type method.
A number of authors have reported that log trends respond well to lithology (e.g., Pirson, 1977;Serra, 1985;Rider, 1986;Milton and Emery, 1996).The use of log trends in lithology interpretation has received much attention in recent years in Taiwan (Lee, 1992;Yu, 1997).
However, no studies have investigated gamm a log trends in groundwater wells.Usually, ground water wells are bigger than petroleum wells in diameter and are composed of unconsolidated Quaternary sediments.In groundwater monitoring of Water Resources Bureau wells, logs of conical wells were taken and the diameter effects should be considered as stated in the preced ing paragraphs.This paper applies log trend method and presents the fades of gamma log trends from three groundwater boreholes in the Choshui fan-delta, Taiwan (Figure 2).All of the fades are compared with core samples in order to evaluate the workability of the method in subsurface hydrogeology investigation.

GEOLOGY SETTING
The wells under study are located in the Choshui fan-delta, central Taiwan (Figure 2).The wells are hydrogeological investigation wells of Taiwan Groundwater Monitoring Network Project.The cores of the wells are about 330m.The Ho-Shing well is in the middle of the fan delta, the Hai-Yuan well is on the tidal flat of the lower fan-delta, and the Ton-Shin well is on the boundary.
The Choshui fan-delta is 60km wide and 40km long.Its apex is in Chukuo, and the mor phology is a westward fan from IOOm to the Taiwan Strait.The subsurface geology is com posed of gravel in east with a transition to sand and mud in west (Figure 3).The deposition age of the upper 0-60m is Holocene and 60-330m is late Pleistocene (CGS,1995).
Natural Gamma(cps)   3. LOGGING PROCEDURE 637 The logging system, "Geologer 3030" model by OYO Corporation, Japan, includes a combination probe, a battery, a set of sheaves, a winch, and a measuring recorder.The logging system measures resistivity, spontaneous potential and natural gamma.The probe descends the hole centrally with a velocity of about 5 m/min and talces one reading every lOcm.Because the drilling mud will emit some gamma rays, a mud scow must be used to clean the drilling mud from the hole before the measurements are talcen.Well logging was undertalcen after core boring and before construction of the casing and screen.The eight basic log trend fades correspond to mud content or mud layer thickness (Figure 6).The contact relationship is gradual or sharp.(1) DU fades is where gamma ray gradually increasing upward, for example with mud content or thickness of mud layer increase upward and sand content or thickness of sand layer decreasing upward.(2) CU fades is where gamma ray gradually decreases upward, for example with mud content or thickness of mud layer decreasing upward and sand content or thickness of sand layer increasing upward.(3) IR fades is where gamma log curve fluctuates, for example with sand and mud interlayer.(4) BW facies is where gamma ray gradual decreases upward and then gradual increases upward, for example with mud content or thickness decreasing upward and then increasing upward.(5) LB fades is low gamma reading, sharp boundaries and no internal change, for example with massive sand layer.( 6) RB fades is high gamma reading, sharp boundaries and no internal change, for example with massive silt or clay layer.(7) RP facies is low gamma reading but includes one or more sharp boundary thin high gamma layer, for example with the massive sand with thin mud layer.( 8) LP fades is high gamma reading but includes one or more sharp boundary thin low gamma layer, for example with the massive mud with thin sand layer.
Basic log trend facies will mix as a superposition curve.For example, the layer between 10 and 30 meters in depth of Ton-Shin well shows a fluctuating IR curve but the gamma reading is also decreasing upward and shows as CU facies (Figure 4).The lithology of this layer is an interlayer of sand and mud, while the mud content and mud thickness is decreasing upward.

DISCUSSION
Table 3 shows eight types of basic gamma log trends and the terminology used in previ ous studies and by the author.The facies BX, CU, DU, BW and IR follow the terminology used by Milton and Emery (1996).However, BX is divided into two facies, and each is com posed of homogeneous sand (RB) and mud (LB), which are usually with a thickness of more than 5m.The LP and RP facies are defined by this study.For example, in the Hai-Yuan well, at a depth of 25-35m, LP facies is massive mud containing a thin sand layer.The two facies can also be found in Ho-Shing well, 85-93m; and in Hai-Yuan well, 71-76m.It is reasonable to include the two facies in basic gamma log trend facies.This paper presents three gamma logs and lithology for Ho-Shing, Hai-Yuan, and Ton Shin wells in the Choshui fan-delta, Taiwan.The author used log trend method to classify the natural gamma logs and compared then with core samples for confidence.Eight basic fades were found in the three logs.Facies DU, CU, IR, BW, LB, and RB are quoted from Milton and Emery (1996), and facies LB and RB are defined by this paper.The log trend was correspond closely to mud content or thickness.

Fig. 1 .
Fig. 1.Casing, caves and borehole diameter variations will shift natural gamm a readings.Natural gamma reading increases down hole when borehole diameter decreases, and decreases when borehole has casing or caves.
and lithology columns of the study wells are shown in Figure 4. Gamma logs are classified into eight basic log trends.
Figure 5 shows centimeter by centimeter correlation of the lithology and log of the Ho-Shing well.The relation indicates that the gamma reading increases with increases in the mud layer thickness.

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Fig. 6.Basic gamma log trend fades and contact.

Table 1 .
Mud content from sieve analysis and gamma log.

Table 3 .
Terminology used in previous studies and this paper.