foo.py

#-------------------------------------------------------------------------------
# Name:        module1
# Purpose:
#
# Author:      woswald
#
# Created:     10/08/2015
# Copyright:   (c) woswald 2015
#-------------------------------------------------------------------------------
#!/usr/bin/env python
import sys
import csv
# change these
inputfile =  r"C:\Users\woswald\Desktop\Test_OG.csv"
outputfile = r"C:\Users\woswald\Desktop\Test_OG_return.csv"
reader = csv.DictReader(open(inputfile))
output = open(outputfile, "w")

# for each row get the api and zone then create new field apiZone
rows = {}
for row in reader:
    api = row['welldata_API']
    zone = row['PROD_ZONE']
    apiZone = (api, zone) # this is a "tuple" you are using it as a key
    if apiZone not in rows:
        rows[apiZone] = {"OW":0, "GW":0, "oil":0, "gas":0, "water":0}
    if row['well type'] == 'OW': # replace 'well type' with the correct column name
        rows[apiZone]["OW"]+=1
    else:
        rows[apiZone]["GW"]+=1
    # add oil, gas, water
    rows[apiZone]['oil']+=row['oil prod']
    rows[apiZone]['gas']+=row['oas prod']
    rows[apiZone]['water']+=row['water prod']

# print header
output.write( "API\tZONE\tAPI_ZONE\n")
for apiZone in sorted(rows.keys()):
    row = rows[apiZone]
    # print out
    wellType='unknown'
    if row['OW'] > row['GW']:
        wellType = 'oil'
    elif row['GW'] > row['OW']:
        wellType = 'gas'
    elif row['oil'] > row['gas']:
        wellType = 'oil'
    elif row['gas'] > row['oil']:
        wellType = 'gas'
    output.write("%s\t%s\t%s\n" % (apiZone[0], apiZone[1], wellType))
output.close()